diff --git a/repos/spack_repo/builtin/packages/fastlowess_cpp/package.py b/repos/spack_repo/builtin/packages/fastlowess_cpp/package.py
new file mode 100644
index 00000000000..85e5d6f3907
--- /dev/null
+++ b/repos/spack_repo/builtin/packages/fastlowess_cpp/package.py
@@ -0,0 +1,72 @@
+# Copyright Spack Project Developers. See COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+#
+# This file is the source of truth for the fastlowess-cpp Spack recipe. It
+# is mirrored into spack/spack-packages by dev/spack_open_pr.sh, which runs
+# on every GitHub release (see .github/workflows/release-cpp.yml).
+#
+# Spack's package API (spack.package, spack_repo.builtin.*) only resolves
+# inside a full Spack installation, which isn't present in this workspace.
+# pyright: reportMissingImports=false, reportUndefinedVariable=false, reportCallIssue=false
+
+from spack.package import *
+from spack_repo.builtin.build_systems.cargo import CargoPackage
+
+
+class FastlowessCpp(CargoPackage):
+    """High-performance LOWESS (Locally Weighted Scatterplot Smoothing)
+    C++ bindings, implemented in Rust."""
+
+    homepage = "https://github.com/thisisamirv/lowess-project"
+    url = (
+        "https://github.com/thisisamirv/lowess-project/archive/refs/tags/v3.1.0.tar.gz"
+    )
+    git = "https://github.com/thisisamirv/lowess-project.git"
+
+    maintainers("thisisamirv")
+
+    license("MIT OR Apache-2.0", checked_by="thisisamirv")
+
+    # version() lines below are appended/updated by dev/spack_release.py on
+    # every release; keep newest first.
+    version("3.1.0", sha256="610a6af65a3e8eaa5483332c256e7ce6c3fe2b7ac3ec0f04e08ecd70bf6abe0f")
+    version(
+        "3.1.0",
+        sha256="610a6af65a3e8eaa5483332c256e7ce6c3fe2b7ac3ec0f04e08ecd70bf6abe0f",
+    )
+
+    depends_on("c", type="build")
+    depends_on("rust@1.89:", type="build")
+
+    build_directory = "bindings/cpp"
+
+    @property
+    def headers(self):
+        return find_headers("fastlowess", root=self.prefix.include, recursive=False)
+
+    @property
+    def libs(self):
+        return find_libraries("libfastlowess_cpp", root=self.prefix, recursive=True)
+
+    def build(self, spec, prefix):
+        with working_dir(self.build_directory):
+            cargo("build", "--release", "--lib")
+
+    def install(self, spec, prefix):
+        with working_dir(self.build_directory):
+            mkdirp(prefix.include)
+            mkdirp(prefix.lib)
+            install(join_path("include", "fastlowess.hpp"), prefix.include)
+            install(join_path("include", "fastlowess.h"), prefix.include)
+
+            release_dir = join_path("target", "release")
+            if spec.satisfies("platform=windows"):
+                mkdirp(prefix.bin)
+                install(join_path(release_dir, "fastlowess_cpp.dll"), prefix.bin)
+                install(join_path(release_dir, "fastlowess_cpp.dll.lib"), prefix.lib)
+            elif spec.satisfies("platform=darwin"):
+                install(join_path(release_dir, "libfastlowess_cpp.dylib"), prefix.lib)
+            else:
+                install(join_path(release_dir, "libfastlowess_cpp.so"), prefix.lib)
+            install(join_path(release_dir, "libfastlowess_cpp.a"), prefix.lib)