diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 72e3559dff28..918b2e53aa7e 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -254,7 +254,7 @@ void NixRepl::mainLoop()
     rl_readline_name = "nix-repl";
     try {
         createDirs(dirOf(historyFile));
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         logWarning(e.info());
     }
 #ifndef USE_READLINE
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index b01d9e237717..f8530195028b 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -1495,7 +1495,7 @@ void LocalDerivationGoal::startDaemon()
                     daemon::processConnection(store, from, to,
                         NotTrusted, daemon::Recursive);
                     debug("terminated daemon connection");
-                } catch (SysError &) {
+                } catch (SystemError &) {
                     ignoreException();
                 }
             });
@@ -1707,7 +1707,7 @@ void LocalDerivationGoal::runChild()
         try {
             if (drv->isBuiltin() && drv->builder == "builtin:fetchurl")
                 netrcData = readFile(settings.netrcFile);
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
 
 #if __linux__
         if (useChroot) {
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 2bd3a2edca05..5cbce0748c2e 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -413,7 +413,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
                     auto env_end = std::sregex_iterator{};
                     for (auto i = std::sregex_iterator{envString.begin(), envString.end(), storePathRegex}; i != env_end; ++i)
                         unchecked[i->str()].emplace(envFile);
-                } catch (SysError & e) {
+                } catch (SystemError & e) {
                     if (errno == ENOENT || errno == EACCES || errno == ESRCH)
                         continue;
                     throw;
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 50584e06cc57..d22ae4ca02e5 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -118,7 +118,7 @@ void loadConfFile()
         try {
             std::string contents = readFile(path);
             globalConfig.applyConfig(contents, path);
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     };
 
     applyConfigFile(settings.nixConfDir + "/nix.conf");
diff --git a/src/libstore/keys.cc b/src/libstore/keys.cc
index 2cc50970f87f..70478e7ad011 100644
--- a/src/libstore/keys.cc
+++ b/src/libstore/keys.cc
@@ -19,7 +19,7 @@ PublicKeys getDefaultPublicKeys()
         try {
             SecretKey secretKey(readFile(secretKeyFile));
             publicKeys.emplace(secretKey.name, secretKey.toPublicKey());
-        } catch (SysError & e) {
+        } catch (SystemError & e) {
             /* Ignore unreadable key files. That's normal in a
                multi-user installation. */
         }
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 0f3c37c8a5f3..5a399c8be3d9 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -276,7 +276,7 @@ LocalStore::LocalStore(const Params & params)
                 [[gnu::unused]] auto res2 = ftruncate(fd.get(), settings.reservedSize);
             }
         }
-    } catch (SysError & e) { /* don't care about errors */
+    } catch (SystemError & e) { /* don't care about errors */
     }
 
     /* Acquire the big fat lock in shared mode to make sure that no
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index a494e6ecce86..78e4f6d86b2c 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -242,7 +242,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
     /* Atomically replace the old file with the new hard link. */
     try {
         renameFile(tempLink, path);
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         if (unlink(tempLink.c_str()) == -1)
             printError("unable to unlink '%1%'", tempLink);
         if (errno == EMLINK) {
diff --git a/src/libstore/remote-fs-accessor.cc b/src/libstore/remote-fs-accessor.cc
index 03e57a565d09..b44edfe890df 100644
--- a/src/libstore/remote-fs-accessor.cc
+++ b/src/libstore/remote-fs-accessor.cc
@@ -87,13 +87,13 @@ std::pair<ref<SourceAccessor>, CanonPath> RemoteFSAccessor::fetch(const CanonPat
             nars.emplace(storePath.hashPart(), narAccessor);
             return {narAccessor, restPath};
 
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
 
         try {
             auto narAccessor = makeNarAccessor(nix::readFile(cacheFile));
             nars.emplace(storePath.hashPart(), narAccessor);
             return {narAccessor, restPath};
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     }
 
     StringSink sink;
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index e2668c673d16..5187e739625f 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -304,7 +304,7 @@ void RootArgs::parseCmdline(const Strings & _cmdline, bool allowShebang)
                 for (auto pos = savedArgs.begin(); pos != savedArgs.end();pos++)
                     cmdline.push_back(*pos);
             }
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     }
     for (auto pos = cmdline.begin(); pos != cmdline.end(); ) {
 
diff --git a/src/libutil/cgroup.cc b/src/libutil/cgroup.cc
index 4c2bf31ffe1a..de83b5ad107b 100644
--- a/src/libutil/cgroup.cc
+++ b/src/libutil/cgroup.cc
@@ -95,7 +95,7 @@ static CgroupStats destroyCgroup(const Path & cgroup, bool returnStats)
                     using namespace std::string_literals;
                     warn("killing stray builder process %d (%s)...",
                         pid, trim(replaceStrings(cmdline, "\0"s, " ")));
-                } catch (SysError &) {
+                } catch (SystemError &) {
                 }
             }
             // FIXME: pid wraparound
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index a3310f4ecb0b..37f5b50c7b3f 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -124,7 +124,7 @@ static void applyConfigInner(const std::string & contents, const std::string & p
                 try {
                     std::string includedContents = readFile(path);
                     applyConfigInner(includedContents, p, parsedContents);
-                } catch (SysError &) {
+                } catch (SystemError &) {
                     // TODO: Do we actually want to ignore this? Or is it better to fail?
                 }
             } else if (!ignoreMissing) {
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 234cbe1f64a5..c91f2e2b33c8 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -178,20 +178,45 @@ MakeError(Error, BaseError);
 MakeError(UsageError, Error);
 MakeError(UnimplementedError, Error);
 
-class SysError : public Error
+/**
+ * To use in catch-blocks.
+ */
+MakeError(SystemError, Error);
+
+/**
+ * POSIX system error, created using `errno`, `strerror` friends.
+ *
+ * Throw this, but do not catch this! Catch `SystemError` instead. This
+ * allows implementations to freely switch between this and `WinError`
+ * without breaking catch blocks.
+ *
+ * @todo Rename this to `PosixError` or similar. At this point Windows
+ * support is too WIP to justify the code churn, but if it is finished
+ * then a better identifier becomes moe worth it.
+ */
+class SysError : public SystemError
 {
 public:
     int errNo;
 
+    /**
+     * Construct using the explicitly-provided error number. `strerror`
+     * will be used to try to add additional information to the message.
+     */
     template<typename... Args>
-    SysError(int errNo_, const Args & ... args)
-        : Error("")
+    SysError(int errNo, const Args & ... args)
+        : SystemError(""), errNo(errNo)
     {
-        errNo = errNo_;
         auto hf = hintfmt(args...);
         err.msg = hintfmt("%1%: %2%", normaltxt(hf.str()), strerror(errNo));
     }
 
+    /**
+     * Construct using the ambient `errno`.
+     *
+     * Be sure to not perform another `errno`-modifying operation before
+     * calling this constructor!
+     */
     template<typename... Args>
     SysError(const Args & ... args)
         : SysError(errno, args ...)
@@ -199,7 +224,9 @@ public:
     }
 };
 
-/** Throw an exception for the purpose of checking that exception handling works; see 'initLibUtil()'.
+/**
+ * Throw an exception for the purpose of checking that exception
+ * handling works; see 'initLibUtil()'.
  */
 void throwExceptionSelfCheck();
 
diff --git a/src/libutil/file-descriptor.cc b/src/libutil/file-descriptor.cc
index 38dd70c8e4c3..692be3383613 100644
--- a/src/libutil/file-descriptor.cc
+++ b/src/libutil/file-descriptor.cc
@@ -231,7 +231,7 @@ void closeMostFDs(const std::set<int> & exceptions)
             }
         }
         return;
-    } catch (SysError &) {
+    } catch (SystemError &) {
     }
 #endif
 
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 183aee2dc6a3..d68ddacc0c17 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -116,7 +116,7 @@ void writeToStderr(std::string_view s)
 {
     try {
         writeFull(STDERR_FILENO, s, false);
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         /* Ignore failing writes to stderr.  We need to ignore write
            errors to ensure that cleanup code that logs to stderr runs
            to completion if the other side of stderr has been closed
diff --git a/src/libutil/namespaces.cc b/src/libutil/namespaces.cc
index a789b321e174..c9ac0c6801cb 100644
--- a/src/libutil/namespaces.cc
+++ b/src/libutil/namespaces.cc
@@ -52,7 +52,7 @@ bool userNamespacesSupported()
 
             auto r = pid.wait();
             assert(!r);
-        } catch (SysError & e) {
+        } catch (SystemError & e) {
             debug("user namespaces do not work on this system: %s", e.msg());
             return false;
         }
@@ -91,7 +91,7 @@ bool mountAndPidNamespacesSupported()
                 return false;
             }
 
-        } catch (SysError & e) {
+        } catch (SystemError & e) {
             debug("mount namespaces do not work on this system: %s", e.msg());
             return false;
         }
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 76b378e18713..316105603cb2 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -53,7 +53,7 @@ void FdSink::writeUnbuffered(std::string_view data)
     written += data.size();
     try {
         writeFull(fd, data);
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         _good = false;
         throw;
     }
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 7b4b1d03103a..b23362b5c9fa 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -20,7 +20,7 @@ void initLibUtil() {
     // When exception handling fails, the message tends to be printed by the
     // C++ runtime, followed by an abort.
     // For example on macOS we might see an error such as
-    // libc++abi: terminating with uncaught exception of type nix::SysError: error: C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.
+    // libc++abi: terminating with uncaught exception of type nix::SystemError: error: C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.
     bool caught = false;
     try {
         throwExceptionSelfCheck();
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index ee2addb720eb..1ad4b387c1a9 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -148,7 +148,7 @@ static void main_nix_build(int argc, char * * argv)
                             args.push_back(word);
                 }
             }
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     }
 
     struct MyArgs : LegacyArgs, MixEvalArgs
diff --git a/src/nix/config-check.cc b/src/nix/config-check.cc
index 410feca2f2ea..8d4717e150b6 100644
--- a/src/nix/config-check.cc
+++ b/src/nix/config-check.cc
@@ -107,7 +107,7 @@ struct CmdConfigCheck : StoreCommand
                     if (profileDir.find("/profiles/") == std::string::npos)
                         dirs.insert(dir);
                 }
-            } catch (SysError &) {}
+            } catch (SystemError &) {}
         }
 
         if (!dirs.empty()) {
diff --git a/tests/unit/libutil/logging.cc b/tests/unit/libutil/logging.cc
index c6dfe63d39b3..8950a26d4e9a 100644
--- a/tests/unit/libutil/logging.cc
+++ b/tests/unit/libutil/logging.cc
@@ -73,7 +73,7 @@ namespace nix {
 
     }
 
-    TEST(logEI, picksUpSysErrorExitCode) {
+    TEST(logEI, picksUpSystemErrorExitCode) {
 
         MakeError(TestError, Error);
         ErrorInfo::programName = std::optional("error-unit-test");
@@ -81,12 +81,12 @@ namespace nix {
         try {
             auto x = readFile(-1);
         }
-        catch (SysError &e) {
+        catch (SystemError &e) {
             testing::internal::CaptureStderr();
             logError(e.info());
             auto str = testing::internal::GetCapturedStderr();
 
-            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SysError --- error-unit-test\x1B[0m\nstatting file: \x1B[33;1mBad file descriptor\x1B[0m\n");
+            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SystemError --- error-unit-test\x1B[0m\nstatting file: \x1B[33;1mBad file descriptor\x1B[0m\n");
         }
     }