diff --git a/src/libstore/unix/build/derivation-builder.cc b/src/libstore/unix/build/derivation-builder.cc
index 3a6f71555..04e8cb176 100644
--- a/src/libstore/unix/build/derivation-builder.cc
+++ b/src/libstore/unix/build/derivation-builder.cc
@@ -18,6 +18,7 @@
 #include "nix/store/user-lock.hh"
 #include "nix/store/globals.hh"
 #include "nix/store/build/derivation-env-desugar.hh"
+#include "nix/util/terminal.hh"
 
 #include <queue>
 
@@ -808,8 +809,7 @@ std::optional<Descriptor> DerivationBuilderImpl::startBuild()
     if (!builderOut)
         throw SysError("opening pseudoterminal master");
 
-    // FIXME: not thread-safe, use ptsname_r
-    std::string slaveName = ptsname(builderOut.get());
+    std::string slaveName = getPtsName(builderOut.get());
 
     if (buildUser) {
         if (chmod(slaveName.c_str(), 0600))
@@ -923,7 +923,7 @@ void DerivationBuilderImpl::prepareSandbox()
 
 void DerivationBuilderImpl::openSlave()
 {
-    std::string slaveName = ptsname(builderOut.get());
+    std::string slaveName = getPtsName(builderOut.get());
 
     AutoCloseFD builderOut = open(slaveName.c_str(), O_RDWR | O_NOCTTY);
     if (!builderOut)
diff --git a/src/libutil/include/nix/util/terminal.hh b/src/libutil/include/nix/util/terminal.hh
index f19de268c..fa71e074e 100644
--- a/src/libutil/include/nix/util/terminal.hh
+++ b/src/libutil/include/nix/util/terminal.hh
@@ -36,4 +36,12 @@ void updateWindowSize();
  */
 std::pair<unsigned short, unsigned short> getWindowSize();
 
+/**
+ * Get the slave name of a pseudoterminal in a thread-safe manner.
+ *
+ * @param fd The file descriptor of the pseudoterminal master
+ * @return The slave device name as a string
+ */
+std::string getPtsName(int fd);
+
 } // namespace nix
diff --git a/src/libutil/terminal.cc b/src/libutil/terminal.cc
index b5765487c..656847487 100644
--- a/src/libutil/terminal.cc
+++ b/src/libutil/terminal.cc
@@ -1,6 +1,7 @@
 #include "nix/util/terminal.hh"
 #include "nix/util/environment-variables.hh"
 #include "nix/util/sync.hh"
+#include "nix/util/error.hh"
 
 #ifdef _WIN32
 #  include <io.h>
@@ -12,6 +13,8 @@
 #endif
 #include <unistd.h>
 #include <widechar_width.h>
+#include <mutex>
+#include <cstdlib> // for ptsname and ptsname_r
 
 namespace {
 
@@ -176,4 +179,29 @@ std::pair<unsigned short, unsigned short> getWindowSize()
     return *windowSize.lock();
 }
 
+std::string getPtsName(int fd)
+{
+#ifdef __APPLE__
+    static std::mutex ptsnameMutex;
+    // macOS doesn't have ptsname_r, use mutex-protected ptsname
+    std::lock_guard<std::mutex> lock(ptsnameMutex);
+    const char * name = ptsname(fd);
+    if (!name) {
+        throw SysError("getting pseudoterminal slave name");
+    }
+    return name;
+#else
+    // Use thread-safe ptsname_r on platforms that support it
+    // PTY names are typically short:
+    // - Linux: /dev/pts/N (where N is usually < 1000)
+    // - FreeBSD: /dev/pts/N
+    // 64 bytes is more than sufficient for any Unix PTY name
+    char buf[64];
+    if (ptsname_r(fd, buf, sizeof(buf)) != 0) {
+        throw SysError("getting pseudoterminal slave name");
+    }
+    return buf;
+#endif
+}
+
 } // namespace nix