diff --git a/include/coap/coap_timer.h b/include/coap/coap_timer.h
index cee8887fd6..d5972f2df2 100644
--- a/include/coap/coap_timer.h
+++ b/include/coap/coap_timer.h
@@ -16,7 +16,7 @@ void coap_free_timer(coap_timer_t *timer);
 
 void coap_timer_set(coap_timer_t *timer, coap_tick_t num_ticks);
 
-void coap_timer_is_set(coap_timer_t *timer);
+char coap_timer_is_set(coap_timer_t *timer);
 
 void coap_timer_unset(coap_timer_t *timer);
 
diff --git a/include/coap/mem.h b/include/coap/mem.h
index e4ef73c1b9..80ab165f3c 100644
--- a/include/coap/mem.h
+++ b/include/coap/mem.h
@@ -38,7 +38,8 @@ typedef enum {
   COAP_PDU,
   COAP_PDU_BUF,
   COAP_RESOURCE,
-  COAP_RESOURCEATTR
+  COAP_RESOURCEATTR,
+  COAP_TIMER
 } coap_memory_tag_t;
 
 #ifndef WITH_LWIP
diff --git a/platform/posix/platform_timer.c b/platform/posix/platform_timer.c
index e8b0637b6e..2a2c468925 100644
--- a/platform/posix/platform_timer.c
+++ b/platform/posix/platform_timer.c
@@ -1,37 +1,124 @@
 #include "coap_timer.h"
 
+#include "mem.h"
+
+#include <string.h>
+#include <sys/time.h> // TODO needs something like HAVE_TIME_H
+#include <signal.h> // TODO needs something like HAVE_SIGNAL_H
+
 #ifdef __GNUC__
 #define UNUSED __attribute__((unused))
 #else
 #define UNUSED
 #endif
 
+struct coap_timer_t;
+
 struct coap_timer_t {
-  int time;
+  coap_timer_t *next_timer;
+  CoapTimerCallback cb;
+  coap_tick_t base_time;
+  coap_tick_t duration;
+  void *data;
 };
 
-void coap_timer_init(void) {
+static struct sigaction timer_sigaction;
+
+static struct coap_timer_t *running_timers;
+
+static void update_itimer(void) {
+  coap_tick_t num_ticks = running_timers->duration - running_timers->base_time;
+
+  struct itimerval it = {
+    .it_value.tv_sec = num_ticks / COAP_TICKS_PER_SECOND,
+    .it_value.tv_usec = num_ticks % COAP_TICKS_PER_SECOND * 1000, // MS -> US
+  };
 
+  setitimer(ITIMER_REAL, &it, NULL);
 }
 
-coap_timer_t *coap_new_timer(CoapTimerCallback cb UNUSED, void *data UNUSED) {
-  return NULL;
+static void sigaction_handler(int sig) {
+  // TODO assert(running_timers);
+  // remove timer from the running list
+  coap_timer_t *timer = running_timers;
+  running_timers = running_timers->next_timer;
+
+  // execute timer callback
+  if (timer->cb) {
+    timer->cb(timer->data);
+  }
+
+  if (running_timers) {
+    // set the itimer again
+    update_itimer();
+  }
+
+  signal(sig, SIG_IGN);
 }
 
-void coap_free_timer(coap_timer_t *timer UNUSED) {
-  return;
+void coap_timer_init(void) {
+  timer_sigaction.sa_sigaction = sigaction_handler;
+  sigaction(SIGALRM, &timer_sigaction, NULL);
 }
 
-void coap_timer_set(coap_timer_t *timer UNUSED, coap_tick_t num_ticks UNUSED) {
-  return;
+coap_timer_t *coap_new_timer(CoapTimerCallback cb, void *data) {
+  coap_timer_t *timer = coap_malloc_type(COAP_TIMER, sizeof(coap_timer_t));
+  memset(timer, 0, sizeof(coap_timer_t));
+  timer->cb = cb;
+  timer->data = data;
+  return timer;
 }
 
-void coap_timer_is_set(coap_timer_t *timer UNUSED) {
-  return;
+void coap_free_timer(coap_timer_t *timer) {
+  // TODO remove from list
+  memset(timer, 0, sizeof(coap_timer_t));
+  coap_free_type(COAP_TIMER, timer);
 }
 
-void coap_timer_unset(coap_timer_t *timer UNUSED) {
-  return;
+void coap_timer_set(coap_timer_t *timer, coap_tick_t num_ticks) {
+  coap_timer_t **cur = &running_timers;
+  while (*cur &&
+         (*cur)->duration - (*cur)->base_time < num_ticks) {
+    cur = &(*cur)->next_timer;
+  }
+  *cur = timer;
+
+  // set basetime
+  coap_ticks(&timer->base_time);
+
+  if (timer == running_timers) {
+    update_itimer();
+  }
+}
+
+char coap_timer_is_set(coap_timer_t *timer) {
+  coap_timer_t *cur = running_timers;
+  while (cur) {
+    if (cur == timer) {
+      return 1;
+    }
+    cur = cur->next_timer;
+  }
+
+  return 0;
 }
 
+void coap_timer_unset(coap_timer_t *timer) {
+  char update_timer = (timer == running_timers);
+
+  coap_timer_t **cur = &running_timers;
+  while (*cur) {
+    if (*cur == timer) {
+      break;
+    }
+    cur = &(*cur)->next_timer;
+  }
+  *cur = (*cur)->next_timer;
+
+  if (update_timer) {
+    update_itimer();
+  }
+
+  return;
+}