diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index ab7b403383dfccfd255f803981186e416f48e52f..6ba9530199e82b05969552214eaf368556342001 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -56,3 +56,10 @@ jobs:
           cargo run -- contract new foobar
           echo "[workspace]" >> foobar/Cargo.toml
           cargo run -- contract build --manifest-path=foobar/Cargo.toml
+
+      - name: Run tests on {{ matrix.platform }}-${{ matrix.toolchain }}
+        # The tests take a long time in the GitHub Actions runner (~30 mins),
+        # hence we run them only on `master`.
+        if: github.ref == 'refs/heads/master'
+        run: |
+          cargo test --verbose --workspace --all-features
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c62a161d0003a9fe2ba4e09c33482c8c6499c985..4e68e7b173ee7af3c4921830f574dd56e2a3b42e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Fixed a Windows issue with contract files in sub-folders - [#313](https://github.com/paritytech/cargo-contract/pull/313)
+
 ## [0.13.0] - 2021-07-22
 
 ### Added
diff --git a/src/cmd/build.rs b/src/cmd/build.rs
index cf2ef906f6ecb8338284d227fa83380d39df24e0..bafa1d31372fbc8ae18a567f443b8dcced5a6d1a 100644
--- a/src/cmd/build.rs
+++ b/src/cmd/build.rs
@@ -1109,6 +1109,41 @@ mod tests_ci_only {
         })
     }
 
+    #[test]
+    fn building_contract_with_source_file_in_subfolder_must_work() {
+        with_new_contract_project(|manifest_path| {
+            // given
+            let path = manifest_path.directory().expect("dir must exist");
+            let old_lib_path = path.join(Path::new("lib.rs"));
+            let new_lib_path = path.join(Path::new("srcfoo")).join(Path::new("lib.rs"));
+            let new_dir_path = path.join(Path::new("srcfoo"));
+            std::fs::create_dir_all(new_dir_path).expect("creating dir must work");
+            std::fs::rename(old_lib_path, new_lib_path).expect("moving file must work");
+
+            let mut manifest =
+                Manifest::new(manifest_path.clone()).expect("creating manifest must work");
+            manifest
+                .set_lib_path("srcfoo/lib.rs")
+                .expect("setting lib path must work");
+            manifest.write(&manifest_path).expect("writing must work");
+
+            // when
+            let res = super::execute(
+                &manifest_path,
+                Verbosity::Default,
+                BuildMode::default(),
+                BuildArtifacts::CheckOnly,
+                UnstableFlags::default(),
+                OptimizationPasses::default(),
+                Default::default(),
+            );
+
+            // then
+            assert!(res.is_ok(), "building contract failed!");
+            Ok(())
+        })
+    }
+
     #[test]
     fn keep_debug_symbols_in_debug_mode() {
         with_new_contract_project(|manifest_path| {
diff --git a/src/workspace/manifest.rs b/src/workspace/manifest.rs
index fba2d0b052014e8f1e8b6da9cb2a0543bc6e5ce8..24f47c0368443c9348c51caa878901da6d7dcc7e 100644
--- a/src/workspace/manifest.rs
+++ b/src/workspace/manifest.rs
@@ -19,9 +19,9 @@ use anyhow::{Context, Result};
 use super::{metadata, Profile};
 use crate::OptimizationPasses;
 
-use std::convert::TryFrom;
 use std::{
     collections::HashSet,
+    convert::TryFrom,
     fs,
     path::{Path, PathBuf},
 };
@@ -248,6 +248,19 @@ impl Manifest {
             .insert("name".into(), value::Value::String(name.into())))
     }
 
+    /// Set the `lib` path to `path`.
+    #[cfg(feature = "test-ci-only")]
+    #[cfg(test)]
+    pub fn set_lib_path(&mut self, path: &str) -> Result<Option<toml::Value>> {
+        Ok(self
+            .toml
+            .get_mut("lib")
+            .ok_or_else(|| anyhow::anyhow!("[lib] section not found"))?
+            .as_table_mut()
+            .ok_or_else(|| anyhow::anyhow!("[lib] should be a table"))?
+            .insert("path".into(), value::Value::String(path.into())))
+    }
+
     /// Set `[profile.release]` lto flag
     pub fn with_profile_release_lto(&mut self, enabled: bool) -> Result<&mut Self> {
         let lto = self
@@ -355,6 +368,10 @@ impl Manifest {
             let path_str = existing_path
                 .as_str()
                 .ok_or_else(|| anyhow::anyhow!("{} should be a string", value_id))?;
+            #[cfg(windows)]
+            // On Windows path separators are `\`, hence we need to replace the `/` in
+            // e.g. `src/lib.rs`.
+            let path_str = &path_str.replace("/", "\\");
             let path = PathBuf::from(path_str);
             if path.is_relative() {
                 let lib_abs = abs_dir.join(path);