diff --git a/src/header/sys_wait/mod.rs b/src/header/sys_wait/mod.rs
--- a/src/header/sys_wait/mod.rs
+++ b/src/header/sys_wait/mod.rs
@@ -4,12 +4,16 @@
 
 use crate::{
     error::ResultExt,
+    header::signal::siginfo_t,
     out::Out,
     platform::{
-        Pal, Sys,
-        types::{c_int, pid_t},
+        ERRNO, Pal, Sys,
+        types::{c_int, c_uint, pid_t},
     },
 };
+
+pub type idtype_t = c_int;
+pub type id_t = c_uint;
 
 pub const WNOHANG: c_int = 1;
 pub const WUNTRACED: c_int = 2;
@@ -24,25 +28,143 @@
 #[allow(overflowing_literals)]
 pub const __WCLONE: c_int = 0x8000_0000;
 
+pub const P_ALL: idtype_t = 0;
+pub const P_PID: idtype_t = 1;
+pub const P_PGID: idtype_t = 2;
+
+pub const CLD_EXITED: c_int = 1;
+pub const CLD_KILLED: c_int = 2;
+pub const CLD_DUMPED: c_int = 3;
+pub const CLD_TRAPPED: c_int = 4;
+pub const CLD_STOPPED: c_int = 5;
+pub const CLD_CONTINUED: c_int = 6;
+
+fn wexitstatus(status: c_int) -> c_int {
+    (status >> 8) & 0xff
+}
+
+fn wtermsig(status: c_int) -> c_int {
+    status & 0x7f
+}
+
+fn wstopsig(status: c_int) -> c_int {
+    wexitstatus(status)
+}
+
+fn wcoredump(status: c_int) -> bool {
+    (status & 0x80) != 0
+}
+
+fn wifexited(status: c_int) -> bool {
+    (status & 0x7f) == 0
+}
+
+fn wifstopped(status: c_int) -> bool {
+    (status & 0xff) == 0x7f
+}
+
+fn wifcontinued(status: c_int) -> bool {
+    status == 0xffff
+}
+
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/wait.html>.
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn wait(stat_loc: *mut c_int) -> pid_t {
     unsafe { waitpid(!0, stat_loc, 0) }
 }
 
-/*
- * TODO: implement idtype_t, id_t, and siginfo_t
- *
- * #[unsafe(no_mangle)]
- * pub unsafe extern "C" fn waitid(
- *     idtype: idtype_t,
- *     id: id_t,
- *     infop: siginfo_t,
- *     options: c_int
- *  ) -> c_int {
- *      unimplemented!();
- *  }
- */
+fn map_waitid_target(idtype: idtype_t, id: id_t) -> Option<pid_t> {
+    match idtype {
+        P_ALL => Some(-1),
+        P_PID => Some(id as pid_t),
+        P_PGID => Some(if id == 0 { 0 } else { -(id as pid_t) }),
+        _ => None,
+    }
+}
+
+fn map_waitid_options(options: c_int) -> Option<c_int> {
+    let interest = options & (WEXITED | WSTOPPED | WCONTINUED);
+    if interest == 0 {
+        return None;
+    }
+
+    let mut waitpid_options = 0;
+    if options & WNOHANG != 0 {
+        waitpid_options |= WNOHANG;
+    }
+    if options & WSTOPPED != 0 {
+        waitpid_options |= WUNTRACED;
+    }
+    if options & WCONTINUED != 0 {
+        waitpid_options |= WCONTINUED;
+    }
+    if options & WNOWAIT != 0 {
+        waitpid_options |= WNOWAIT;
+    }
+
+    Some(waitpid_options)
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn waitid(
+    idtype: idtype_t,
+    id: id_t,
+    infop: *mut siginfo_t,
+    options: c_int,
+) -> c_int {
+    if infop.is_null() {
+        ERRNO.set(crate::header::errno::EFAULT);
+        return -1;
+    }
+
+    let Some(pid_target) = map_waitid_target(idtype, id) else {
+        ERRNO.set(crate::header::errno::EINVAL);
+        return -1;
+    };
+    let Some(waitpid_options) = map_waitid_options(options) else {
+        ERRNO.set(crate::header::errno::EINVAL);
+        return -1;
+    };
+
+    let mut status = 0;
+    let pid = Sys::waitpid(pid_target, Some(Out::from_mut(&mut status)), waitpid_options)
+        .or_minus_one_errno();
+    if pid < 0 {
+        return -1;
+    }
+
+    unsafe {
+        *infop = core::mem::zeroed();
+    }
+    if pid == 0 {
+        return 0;
+    }
+
+    unsafe {
+        (*infop).si_pid = pid;
+        (*infop).si_signo = crate::header::signal::SIGCHLD as c_int;
+        (*infop).si_errno = 0;
+        if wifexited(status) {
+            (*infop).si_code = CLD_EXITED;
+            (*infop).si_status = wexitstatus(status);
+        } else if wifstopped(status) {
+            (*infop).si_code = CLD_STOPPED;
+            (*infop).si_status = wstopsig(status);
+        } else if wifcontinued(status) {
+            (*infop).si_code = CLD_CONTINUED;
+            (*infop).si_status = crate::header::signal::SIGCONT as c_int;
+        } else {
+            (*infop).si_status = wtermsig(status);
+            (*infop).si_code = if wcoredump(status) {
+                CLD_DUMPED
+            } else {
+                CLD_KILLED
+            };
+        }
+    }
+
+    0
+}
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/waitpid.html>.
 #[unsafe(no_mangle)]
diff --git a/tests/Makefile.tests.mk b/tests/Makefile.tests.mk
--- a/tests/Makefile.tests.mk
+++ b/tests/Makefile.tests.mk
@@ -312,6 +312,7 @@
 	grp/getgrgid_r \
 	grp/getgrnam_r \
 	grp/gr_iter \
+	waitid \
 	waitpid \
 	waitpid_multiple \
 	$(FAILING_TESTS)
diff --git a/tests/waitid.c b/tests/waitid.c
new file mode 100644
--- /dev/null
+++ b/tests/waitid.c
@@ -0,0 +1,50 @@
+#include <assert.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "test_helpers.h"
+
+static void wait_until_child_exits(pid_t pid) {
+    siginfo_t info;
+    for (int i = 0; i < 50; ++i) {
+        info.si_pid = 0;
+        int ret = waitid(P_PID, pid, &info, WEXITED | WNOHANG | WNOWAIT);
+        ERROR_IF(waitid, ret, == -1);
+        if (info.si_pid == pid) {
+            assert(info.si_code == CLD_EXITED);
+            assert(info.si_status == 42);
+            return;
+        }
+        usleep(10000);
+    }
+    assert(!"waitid never observed child exit");
+}
+
+int main(void) {
+    pid_t pid = fork();
+    ERROR_IF(fork, pid, == -1);
+
+    if (pid == 0) {
+        usleep(50000);
+        _Exit(42);
+    }
+
+    siginfo_t info;
+    info.si_pid = 0;
+    int ret = waitid(P_PID, pid, &info, WEXITED | WNOHANG | WNOWAIT);
+    ERROR_IF(waitid, ret, == -1);
+    assert(info.si_pid == 0);
+
+    wait_until_child_exits(pid);
+
+    int status = 0;
+    pid_t waited = waitpid(pid, &status, 0);
+    ERROR_IF(waitpid, waited, == -1);
+    assert(waited == pid);
+    assert(WIFEXITED(status));
+    assert(WEXITSTATUS(status) == 42);
+
+    return EXIT_SUCCESS;
+}