diff --git a/include/pup/platform/file_io.hpp b/include/pup/platform/file_io.hpp
index 1ed57fa2..a994cf99 100644
--- a/include/pup/platform/file_io.hpp
+++ b/include/pup/platform/file_io.hpp
@@ -102,6 +102,8 @@ auto write_file(std::string_view path, std::string_view data) -> Result<void>;
 
 // Directory traversal.
 //
+// Entries come back in name order — readdir order is a filesystem artifact.
+//
 // A DirEntry's `name` is a view into the `names` arena of the DirEntries that
 // produced it — valid until the next read_directory on that DirEntries, or its
 // destruction. In a walk, the `rel_path` handed to the visitor is likewise a
diff --git a/src/platform/file_io-posix.cpp b/src/platform/file_io-posix.cpp
index d052fab1..5509907a 100644
--- a/src/platform/file_io-posix.cpp
+++ b/src/platform/file_io-posix.cpp
@@ -9,6 +9,7 @@
 #include "pup/platform/file_io.hpp"
 #include "pup/platform/sys.hpp"
 
+#include <algorithm>
 #include <cerrno>
 #include <cstdint>
 #include <cstring>
@@ -630,6 +631,7 @@ auto read_directory(std::string_view path, DirEntries& out) -> Result<void>
     for (auto i = std::size_t { 0 }; i < out.entries.size(); ++i) {
         out.entries[i].name = base.substr(offsets[i], offsets[i + 1U] - offsets[i]);
     }
+    std::ranges::sort(out.entries, {}, &DirEntry::name);
     return {};
 }
 
diff --git a/src/platform/file_io-win32.cpp b/src/platform/file_io-win32.cpp
index 29cab68c..e9faf01c 100644
--- a/src/platform/file_io-win32.cpp
+++ b/src/platform/file_io-win32.cpp
@@ -8,6 +8,8 @@
 #include "pup/core/string_pool.hpp"
 #include "pup/platform/file_io.hpp"
 
+#include <algorithm>
+
 #include <windows.h>
 
 namespace pup::platform {
@@ -640,6 +642,7 @@ auto read_directory(std::string_view path, DirEntries& out) -> Result<void>
     for (auto i = std::size_t { 0 }; i < out.entries.size(); ++i) {
         out.entries[i].name = base.substr(offsets[i], offsets[i + 1] - offsets[i]);
     }
+    std::ranges::sort(out.entries, {}, &DirEntry::name);
     return {};
 }
 
diff --git a/test/unit/test_platform_file_io.cpp b/test/unit/test_platform_file_io.cpp
index ca71ab01..2ce2148e 100644
--- a/test/unit/test_platform_file_io.cpp
+++ b/test/unit/test_platform_file_io.cpp
@@ -7,12 +7,14 @@
 #include "pup/core/string_pool.hpp"
 #include "pup/platform/file_io.hpp"
 
+#include <array>
 #include <cstring>
 #include <filesystem>
 #include <fstream>
 #include <set>
 #include <string>
 #include <utility>
+#include <vector>
 
 #ifndef _WIN32
 #include <pthread.h>
@@ -417,3 +419,62 @@ TEST_CASE("walk_directory descends deep trees without a per-frame stack blowup",
     REQUIRE(args.saw_leaf);
 }
 #endif
+
+SCENARIO("read_directory returns entries in name order", "[platform][file_io][walk]")
+{
+    GIVEN("a directory whose entries were created in reverse-lexicographic order")
+    {
+        // Mixed case on purpose: NTFS enumerates case-insensitively, so an
+        // all-lowercase set would already arrive sorted and pin nothing there.
+        auto const names = std::array<std::string_view, 4> {
+            "Bravo", "Zulu", "alpha", "mike"
+        };
+        auto dir = TempDir { "order" };
+        for (auto i = names.size(); i-- > 0;) {
+            dir.write(names[i], "x");
+        }
+
+        WHEN("the directory is read")
+        {
+            auto listing = pup::platform::DirEntries {};
+            REQUIRE(pup::platform::read_directory(dir.path().string(), listing).has_value());
+
+            THEN("the entries are in name order")
+            {
+                auto seen = std::vector<std::string_view> {};
+                for (auto const& entry : listing.entries) {
+                    seen.push_back(entry.name);
+                }
+                REQUIRE(seen == std::vector<std::string_view>(names.begin(), names.end()));
+            }
+        }
+    }
+}
+
+SCENARIO("walk_directory visits every level in name order", "[platform][file_io][walk]")
+{
+    GIVEN("a nested tree created in reverse-lexicographic order")
+    {
+        auto dir = TempDir { "walk_order" };
+        for (auto name : std::array<std::string_view, 4> { "alpha.txt", "sub/bravo.txt", "sub/Yankee.txt", "Zulu.txt" }) {
+            dir.write(name, "x");
+        }
+
+        WHEN("the tree is walked")
+        {
+            auto seen = std::vector<std::string> {};
+            auto r = pup::platform::walk_directory(
+                dir.path().string(),
+                [&](pup::platform::DirEntry const& /*entry*/, std::string_view rel_path) -> bool {
+                    seen.emplace_back(rel_path);
+                    return true;
+                });
+
+            THEN("each directory is enumerated in name order, depth first")
+            {
+                REQUIRE(r.has_value());
+                REQUIRE(seen == std::vector<std::string> { "Zulu.txt", "alpha.txt", "sub", "sub/Yankee.txt", "sub/bravo.txt" });
+            }
+        }
+    }
+}