diff --git a/cpu/cc2538/include/periph_cpu.h b/cpu/cc2538/include/periph_cpu.h
index 034bfc5fd4..05cac34c14 100644
--- a/cpu/cc2538/include/periph_cpu.h
+++ b/cpu/cc2538/include/periph_cpu.h
@@ -332,6 +332,24 @@ typedef gpio_t adc_conf_t;
 #define SOCADC_12_BIT_RSHIFT    (4U) /**< Mask for getting data(12 bits ENOB) */
 /** @} */
 
+/**
+ * @name RTT configuration
+ * @{
+ */
+#define RTT_DEV             SMWDTHROSC
+#define RTT_IRQ             SM_TIMER_ALT_IRQn
+#define RTT_IRQ_PRIO        1
+#define RTT_ISR             isr_sleepmode
+#define RTT_MAX_VALUE       (0xffffffff)
+#if SYS_CTRL_OSC32K_USE_XTAL
+/* Frequency of XOSC off by default */
+#define RTT_FREQUENCY       (32768U)    /* in Hz. For changes see `rtt.c` */
+#else
+/* Frequency of RCOSC on by default */
+#define RTT_FREQUENCY       (32753U)    /* in Hz. For changes see `rtt.c` */
+#endif
+/** @} */
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/cpu/cc2538/periph/rtt.c b/cpu/cc2538/periph/rtt.c
new file mode 100644
index 0000000000..a393e2258a
--- /dev/null
+++ b/cpu/cc2538/periph/rtt.c
@@ -0,0 +1,142 @@
+/*
+ * Copyright (C) 2019 Inria
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     cpu_cc2538
+ * @ingroup     drivers_periph_rtt
+ * @{
+ *
+ * @file
+ * @brief       RTT implementation sleepmode timer
+ *
+ * @author      Francisco Molina <francois-xavier.molina@inria.fr>
+ *
+ * @}
+ */
+
+#include <stdint.h>
+
+#include "cpu.h"
+#include "periph/rtt.h"
+
+#define SMWDTHROSC_STLOAD_STLOAD_MASK  (0x00000001)
+
+/* allocate memory for alarm and overflow callbacks + args */
+static rtt_cb_t alarm_cb = NULL;
+static void *alarm_arg;
+static rtt_cb_t overflow_cb = NULL;
+static void *overflow_arg;
+
+static inline void _rtt_irq_enable(void)
+{
+    NVIC_SetPriority(SM_TIMER_ALT_IRQn, RTT_IRQ_PRIO);
+    NVIC_EnableIRQ(SM_TIMER_ALT_IRQn);
+}
+
+static inline void _rtt_irq_disable(void)
+{
+    NVIC_DisableIRQ(SM_TIMER_ALT_IRQn);
+}
+
+void rtt_poweron(void)
+{
+    /* sleepmode timer is always running so only enable IRQ */
+    _rtt_irq_enable();
+}
+
+void rtt_poweroff(void)
+{
+    /* sleepmode timer is always running so only disable IRQ */
+    _rtt_irq_disable();
+}
+
+void rtt_init(void)
+{
+    rtt_poweron();
+}
+
+uint32_t rtt_get_counter(void)
+{
+    return ((SMWDTHROSC_ST0 & 0xFF) |
+            ((SMWDTHROSC_ST1 & 0xFF) << 8) |
+            ((SMWDTHROSC_ST2 & 0xFF) << 16) |
+            ((SMWDTHROSC_ST3 & 0xFF) << 24));
+}
+
+void rtt_set_alarm(uint32_t alarm, rtt_cb_t cb, void *arg)
+{
+    assert(cb && !(alarm & ~RTT_MAX_VALUE));
+
+    unsigned irq = irq_disable();
+
+    /* set alarm value */
+    while (!(SMWDTHROSC_STLOAD & SMWDTHROSC_STLOAD_STLOAD_MASK)) {}
+    SMWDTHROSC_ST3 = (alarm >> 24) & 0xFF;
+    SMWDTHROSC_ST2 = (alarm >> 16) & 0xFF;
+    SMWDTHROSC_ST1 = (alarm >>  8) & 0xFF;
+    SMWDTHROSC_ST0 = alarm & 0xFF;
+
+    /* set callback*/
+    alarm_cb = cb;
+    alarm_arg = arg;
+
+    irq_restore(irq);
+}
+
+void rtt_clear_alarm(void)
+{
+    unsigned irq = irq_disable();
+
+    alarm_cb = NULL;
+    alarm_arg = NULL;
+    irq_restore(irq);
+}
+
+void rtt_set_overflow_cb(rtt_cb_t cb, void *arg)
+{
+    unsigned irq = irq_disable();
+
+    /* set threshold to RTT_MAX_VALUE */
+    while (!(SMWDTHROSC_STLOAD & SMWDTHROSC_STLOAD_STLOAD_MASK)) {}
+    SMWDTHROSC_ST3 = (RTT_MAX_VALUE >> 24) & 0xFF;
+    SMWDTHROSC_ST2 = (RTT_MAX_VALUE >> 16) & 0xFF;
+    SMWDTHROSC_ST1 = (RTT_MAX_VALUE >>  8) & 0xFF;
+    SMWDTHROSC_ST0 = RTT_MAX_VALUE & 0xFF;
+
+    /* set callback*/
+    overflow_cb = cb;
+    overflow_arg = arg;
+
+    irq_restore(irq);
+}
+
+void rtt_clear_overflow_cb(void)
+{
+    unsigned irq = irq_disable();
+
+    overflow_cb = NULL;
+    overflow_arg = NULL;
+    irq_restore(irq);
+}
+
+void isr_sleepmode(void)
+{
+    if (alarm_cb) {
+        /* 'consume' the callback (as it might be set again in the cb) */
+        rtt_cb_t tmp = alarm_cb;
+        alarm_cb = NULL;
+        tmp(alarm_arg);
+    }
+    else if (overflow_cb) {
+        /* 'consume' the callback (as it might be set again in the cb) */
+        rtt_cb_t tmp = overflow_cb;
+        overflow_cb = NULL;
+        tmp(overflow_arg);
+    }
+    cortexm_isr_end();
+}