diff --git a/zephyr/ra/CMakeLists.txt b/zephyr/ra/CMakeLists.txt
index be023cc6..701b28f5 100644
--- a/zephyr/ra/CMakeLists.txt
+++ b/zephyr/ra/CMakeLists.txt
@@ -15,3 +15,5 @@ zephyr_library_sources_ifdef(CONFIG_USE_RA_FSP_SSI
     portable/src/rp_ssi/rp_ssi.c)
 zephyr_library_sources_ifdef(CONFIG_USE_RA_FSP_I3C
     portable/src/rp_i3c/rp_i3c.c)
+zephyr_library_sources_ifdef(CONFIG_USE_RA_FSP_ADC
+    portable/src/rp_adc/rp_adc.c)
diff --git a/zephyr/ra/portable/inc/rp_adc.h b/zephyr/ra/portable/inc/rp_adc.h
new file mode 100644
index 00000000..b7b2194c
--- /dev/null
+++ b/zephyr/ra/portable/inc/rp_adc.h
@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2020 - 2025 Renesas Electronics Corporation and/or its affiliates
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef RP_ADC_H
+#define RP_ADC_H
+
+/***********************************************************************************************************************
+ * Includes
+ **********************************************************************************************************************/
+#include "r_adc_api.h"
+#include "r_adc.h"
+
+/* Common macro for FSP header files. There is also a corresponding FSP_FOOTER macro at the end of this file. */
+FSP_HEADER
+
+/*****************************************************************************************************************//**
+ * @ingroup ADC
+ * @{
+ ********************************************************************************************************************/
+
+/***********************************************************************************************************************
+ * Macro definitions
+ **********************************************************************************************************************/
+
+/* Definitions for unspecificed value of sampling time. */
+#define UNSPECIFIED 0xFFFFFFFFu
+
+/* Number of microseconds in one second (1 s = 1,000,000 µs) */
+#define ADC_PRV_USEC_PER_SEC                        (1000000U)
+
+/* Number of Hertz in one kilohertz (1 kHz = 1,000 Hz) */
+#define ADC_PRV_HZ_PER_KHZ                          (1000U)
+
+/***********************************************************************************************************************
+ * Typedef definitions
+ **********************************************************************************************************************/
+
+/**********************************************************************************************************************
+ * Exported global variables
+ **********************************************************************************************************************/
+
+/**********************************************************************************************************************
+ * Public Function Prototypes
+ **********************************************************************************************************************/
+
+fsp_err_t RP_ADC_SampleStateCalculation(uint32_t sampling_time_ns, uint32_t *p_sample_count);
+
+/* Common macro for FSP header files. There is also a corresponding FSP_HEADER macro at the top of this file. */
+FSP_FOOTER
+
+#endif
+
+/*******************************************************************************************************************//**
+ * @} (end ingroup ADC)
+ **********************************************************************************************************************/
diff --git a/zephyr/ra/portable/src/rp_adc/rp_adc.c b/zephyr/ra/portable/src/rp_adc/rp_adc.c
new file mode 100644
index 00000000..88b12143
--- /dev/null
+++ b/zephyr/ra/portable/src/rp_adc/rp_adc.c
@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2025 Renesas Electronics Corporation and/or its affiliates
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+/***********************************************************************************************************************
+ * Includes
+ **********************************************************************************************************************/
+#include "r_adc.h"
+#include "r_adc_cfg.h"
+#include "rp_adc.h"
+
+/***********************************************************************************************************************
+ * Macro definitions
+ **********************************************************************************************************************/
+
+/*******************************************************************************************************************//**
+ * Calculate ADC sample state count based on desired sampling time (in ns).
+ *
+ * @param[in]  sampling_time_ns   Desired sampling time in nanoseconds, or UNSPECIFIED to use BSP default.
+ * @param[out] p_sample_count     Pointer to store calculated sample state count.
+ *
+ * @retval FSP_SUCCESS                    Calculation successful.
+ * @retval FSP_ERR_INVALID_ARGUMENT       p_sample_count is NULL.
+ * @retval FSP_ERR_INVALID_HW_CONDITION   ADC clock frequency not available.
+ **********************************************************************************************************************/
+fsp_err_t RP_ADC_SampleStateCalculation(uint32_t sampling_time_ns, uint32_t *p_sample_count)
+{
+	if (NULL == p_sample_count) {
+		return FSP_ERR_INVALID_ARGUMENT;
+	}
+
+	/* Get ADC clock frequency */
+	uint32_t adc_clk_hz = R_FSP_SystemClockHzGet(BSP_FEATURE_ADC_CLOCK_SOURCE);
+	if (0u == adc_clk_hz) {
+		return FSP_ERR_INVALID_HW_CONDITION;
+	}
+
+	/* If user did not specify a sampling time, use the BSP-feature minimum value for ADC */
+	if (UNSPECIFIED == sampling_time_ns) {
+		sampling_time_ns = BSP_FEATURE_ADC_SENSOR_MIN_SAMPLING_TIME;
+	}
+
+	/* Calculate number of sample states with round-up */
+	uint32_t sample_states =
+		((sampling_time_ns * (adc_clk_hz / ADC_PRV_HZ_PER_KHZ)) / ADC_PRV_USEC_PER_SEC) +
+		1U;
+
+	/* Ensure the state count is not smaller than the minimum allowed */
+	if (sample_states < ADC_SAMPLE_STATE_COUNT_MIN) {
+		sample_states = ADC_SAMPLE_STATE_COUNT_MIN;
+	}
+
+	/* Clamp to hardware maximum (8-bit register, so max 255) */
+	if (sample_states > ADC_SAMPLE_STATE_COUNT_MAX) {
+		sample_states = ADC_SAMPLE_STATE_COUNT_MAX;
+	}
+
+	*p_sample_count = sample_states;
+
+	return FSP_SUCCESS;
+}