diff --git a/.travis.yml b/.travis.yml
index 0ac4a21..716757b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -91,13 +91,14 @@ matrix:
         - cargo test --benches
         # Check that setting various features does not break the build
         - cargo build --features=std
+        - cargo build --features=custom
         # remove cached documentation, otherwise files from previous PRs can get included
         - rm -rf target/doc
-        - cargo doc --no-deps --features=std
+        - cargo doc --no-deps --features=std,custom
         - cargo deadlinks --dir target/doc
         # also test minimum dependency versions are usable
         - cargo generate-lockfile -Z minimal-versions
-        - cargo test --features=std
+        - cargo test --features=std,custom
 
     - <<: *nightly_and_docs
       name: "OSX, nightly, docs"
diff --git a/Cargo.toml b/Cargo.toml
index 88dd1f3..1a76212 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -36,10 +36,12 @@ wasm-bindgen-test = "0.2"
 
 [features]
 std = []
+# Feature to enable custom RNG implementations
+custom = []
 # Unstable feature to support being a libstd dependency
 rustc-dep-of-std = ["compiler_builtins", "core"]
 # Unstable feature for testing
 test-in-browser = ["wasm-bindgen"]
 
 [package.metadata.docs.rs]
-features = ["std"]
+features = ["std", "custom"]
diff --git a/src/custom.rs b/src/custom.rs
new file mode 100644
index 0000000..11abf0c
--- /dev/null
+++ b/src/custom.rs
@@ -0,0 +1,45 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! An implementation which calls out to an externally defined function.
+use crate::Error;
+use core::num::NonZeroU32;
+
+/// Register a function to be invoked by `getrandom` on custom targets.
+///
+/// This function will only be invoked on targets not supported by `getrandom`.
+/// This prevents crate dependencies from either inadvertently or maliciously
+/// overriding the secure RNG implementations in `getrandom`.
+///
+/// *This API requires the following crate features to be activated: `custom`*
+#[macro_export]
+macro_rules! register_custom_getrandom {
+    ($path:path) => {
+        // We use an extern "C" function to get the guarantees of a stable ABI.
+        #[no_mangle]
+        extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
+            let slice = unsafe { ::std::slice::from_raw_parts_mut(dest, len) };
+            match $path(slice) {
+                Ok(()) => 0,
+                Err(e) => e.code().get(),
+            }
+        }
+    };
+}
+
+#[allow(dead_code)]
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    extern "C" {
+        fn __getrandom_custom(dest: *mut u8, len: usize) -> u32;
+    }
+    let ret = unsafe { __getrandom_custom(dest.as_mut_ptr(), dest.len()) };
+    match NonZeroU32::new(ret) {
+        None => Ok(()),
+        Some(code) => Err(Error::from(code)),
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 1ce321e..7612ef0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -134,7 +134,10 @@ extern crate cfg_if;
 
 mod error;
 mod util;
-
+// To prevent a breaking change when targets are added, we always export the
+// register_custom_getrandom macro, so old Custom RNG crates continue to build.
+#[cfg(feature = "custom")]
+mod custom;
 #[cfg(feature = "std")]
 mod error_impls;
 
@@ -201,6 +204,8 @@ cfg_if! {
                 ");
             }
         }
+    } else if #[cfg(feature = "custom")] {
+        use custom as imp;
     } else {
         compile_error!("\
             target is not supported, for more information see: \