diff --git test/unit/main.cpp test/unit/main.cpp
index d53e4236..b6981989 100644
--- test/unit/main.cpp
+++ test/unit/main.cpp
@@ -27,10 +27,101 @@
  ****************************************************************************/
 
 #include <locale>
+#include <memory>
+#include <string.h>
+
+#include <filesystem>
+#include <iostream>
+#include <string>
+#include <vector>
 
 #include "gtest_include.h"
+#include "rules_cc/cc/runfiles/runfiles.h"
+
+namespace fs = std::filesystem;
+using rules_cc::cc::runfiles::Runfiles;
+
+namespace {
+
+bool maybeSetupProjEnvironment(int *argc, char ***argv) {
+    std::string proj_ini_rloc;
+    std::string source_data_marker_rloc;
+
+    auto parseArg = [](const char *prefix, const std::string &arg,
+                       std::string *value) {
+        if (arg.rfind(prefix, 0) != 0) {
+            return false;
+        }
+        *value = arg.substr(strlen(prefix));
+        return true;
+    };
+
+    static std::vector<char *> filtered_argv;
+    filtered_argv.clear();
+    filtered_argv.reserve(static_cast<size_t>(*argc) + 1);
+    filtered_argv.push_back((*argv)[0]);
+    for (int i = 1; i < *argc; ++i) {
+        const std::string arg((*argv)[i]);
+        if (!parseArg("--proj-test-proj-ini=", arg, &proj_ini_rloc) &&
+            !parseArg("--proj-test-source-data-marker=", arg,
+                      &source_data_marker_rloc)) {
+            filtered_argv.push_back((*argv)[i]);
+        }
+    }
+    filtered_argv.push_back(nullptr);
+    *argc = static_cast<int>(filtered_argv.size()) - 1;
+    *argv = filtered_argv.data();
+
+    if (proj_ini_rloc.empty()) {
+        return true;
+    }
+
+    std::string error;
+    std::unique_ptr<Runfiles> runfiles(Runfiles::CreateForTest(&error));
+    if (!runfiles) {
+        std::cerr << error << std::endl;
+        return false;
+    }
+
+    const auto proj_ini = fs::path(runfiles->Rlocation(proj_ini_rloc));
+    if (proj_ini.empty()) {
+        std::cerr << "failed to resolve proj.ini" << std::endl;
+        return false;
+    }
+
+    const auto proj_data = proj_ini.parent_path();
+    if (setenv("PROJ_DATA", proj_data.c_str(), 1) != 0 ||
+        setenv("PROJ_LIB", proj_data.c_str(), 1) != 0 ||
+        setenv("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY", "YES", 1) != 0) {
+        std::cerr << "failed to set PROJ runtime environment" << std::endl;
+        return false;
+    }
+
+    if (!source_data_marker_rloc.empty()) {
+        const auto source_data_marker =
+            fs::path(runfiles->Rlocation(source_data_marker_rloc));
+        if (source_data_marker.empty()) {
+            std::cerr << "failed to resolve PROJ source data marker" << std::endl;
+            return false;
+        }
+        if (setenv("PROJ_SOURCE_DATA",
+                   source_data_marker.parent_path().parent_path().c_str(), 1) !=
+            0) {
+            std::cerr << "failed to set PROJ_SOURCE_DATA" << std::endl;
+            return false;
+        }
+    }
+
+    return true;
+}
+
+}  // namespace
 
 GTEST_API_ int main(int argc, char **argv) {
+    if (!maybeSetupProjEnvironment(&argc, &argv)) {
+        return 1;
+    }
+
     // Use a potentially non-C locale to make sure we are robust
     setlocale(LC_ALL, "");