diff --git a/homeassistant/components/nightscout/config_flow.py b/homeassistant/components/nightscout/config_flow.py
index 3000d652e46f87..2590d0cdacd5b0 100644
--- a/homeassistant/components/nightscout/config_flow.py
+++ b/homeassistant/components/nightscout/config_flow.py
@@ -7,14 +7,20 @@
 import voluptuous as vol
 
 from homeassistant import config_entries, exceptions
-from homeassistant.const import CONF_API_KEY, CONF_URL
+from homeassistant.const import CONF_API_KEY, CONF_UNIT_OF_MEASUREMENT, CONF_URL
 
-from .const import DOMAIN  # pylint:disable=unused-import
+from .const import DOMAIN, MGDL, MMOL  # pylint:disable=unused-import
 from .utils import hash_from_url
 
 _LOGGER = logging.getLogger(__name__)
 
-DATA_SCHEMA = vol.Schema({vol.Required(CONF_URL): str, vol.Optional(CONF_API_KEY): str})
+DATA_SCHEMA = vol.Schema(
+    {
+        vol.Required(CONF_URL): str,
+        vol.Optional(CONF_API_KEY): str,
+        vol.Optional(CONF_UNIT_OF_MEASUREMENT, default=MGDL): vol.In([MGDL, MMOL]),
+    }
+)
 
 
 async def _validate_input(data):
diff --git a/homeassistant/components/nightscout/const.py b/homeassistant/components/nightscout/const.py
index 4bb96a94c2971f..fec72b00570d22 100644
--- a/homeassistant/components/nightscout/const.py
+++ b/homeassistant/components/nightscout/const.py
@@ -6,3 +6,8 @@
 ATTR_DATE = "date"
 ATTR_DELTA = "delta"
 ATTR_DIRECTION = "direction"
+
+MMOL_TO_MGDL = 18
+
+MGDL = "mg/dL"
+MMOL = "mmol/L"
diff --git a/homeassistant/components/nightscout/sensor.py b/homeassistant/components/nightscout/sensor.py
index f4ff14d7b2a7ea..fd77197bf3f880 100644
--- a/homeassistant/components/nightscout/sensor.py
+++ b/homeassistant/components/nightscout/sensor.py
@@ -8,10 +8,20 @@
 from py_nightscout import Api as NightscoutAPI
 
 from homeassistant.config_entries import ConfigEntry
+from homeassistant.const import CONF_UNIT_OF_MEASUREMENT
 from homeassistant.core import HomeAssistant
 from homeassistant.helpers.entity import Entity
 
-from .const import ATTR_DATE, ATTR_DELTA, ATTR_DEVICE, ATTR_DIRECTION, DOMAIN
+from .const import (
+    ATTR_DATE,
+    ATTR_DELTA,
+    ATTR_DEVICE,
+    ATTR_DIRECTION,
+    DOMAIN,
+    MGDL,
+    MMOL,
+    MMOL_TO_MGDL,
+)
 
 SCAN_INTERVAL = timedelta(minutes=1)
 
@@ -27,20 +37,23 @@ async def async_setup_entry(
 ) -> None:
     """Set up the Glucose Sensor."""
     api = hass.data[DOMAIN][entry.entry_id]
-    async_add_entities([NightscoutSensor(api, "Blood Sugar", entry.unique_id)], True)
+    uom = entry.data.get(CONF_UNIT_OF_MEASUREMENT, MGDL)
+    async_add_entities(
+        [NightscoutSensor(api, "Blood Sugar", entry.unique_id, uom)], True
+    )
 
 
 class NightscoutSensor(Entity):
     """Implementation of a Nightscout sensor."""
 
-    def __init__(self, api: NightscoutAPI, name, unique_id):
+    def __init__(self, api: NightscoutAPI, name, unique_id, uom):
         """Initialize the Nightscout sensor."""
         self.api = api
         self._unique_id = unique_id
         self._name = name
         self._state = None
         self._attributes = None
-        self._unit_of_measurement = "mg/dL"
+        self._unit_of_measurement = uom
         self._icon = "mdi:cloud-question"
         self._available = False
 
@@ -94,7 +107,13 @@ async def async_update(self):
                 ATTR_DELTA: value.delta,
                 ATTR_DIRECTION: value.direction,
             }
-            self._state = value.sgv
+            if self._unit_of_measurement == MMOL:
+                self._state = "%.1f" % (value.sgv / MMOL_TO_MGDL)
+                self._attributes[ATTR_DELTA] = "%.1f" % (
+                    self._attributes[ATTR_DELTA] / MMOL_TO_MGDL
+                )
+            else:
+                self._state = value.sgv
             self._icon = self._parse_icon()
         else:
             self._available = False
diff --git a/homeassistant/components/nightscout/strings.json b/homeassistant/components/nightscout/strings.json
index 2240bcec02b571..86a223d23ab846 100644
--- a/homeassistant/components/nightscout/strings.json
+++ b/homeassistant/components/nightscout/strings.json
@@ -4,10 +4,11 @@
         "step": {
             "user": {
                 "title": "Enter your Nightscout server information.",
-                "description": "- URL: the address of your nightscout instance. I.e.: https://myhomeassistant.duckdns.org:5423\n- API Key (Optional): Only use if your instance is protected (auth_default_roles != readable).",
+                "description": "- URL: the address of your nightscout instance. e.g.: https://myhomeassistant.duckdns.org:5423\n- API Key (Optional): Only use if your instance is protected (auth_default_roles != readable).\n- Unit of Measurement: Choose between mg/dL or mmol/L.",
                 "data": {
                     "url": "[%key:common::config_flow::data::url%]",
-                    "api_key": "[%key:common::config_flow::data::api_key%]"
+                    "api_key": "[%key:common::config_flow::data::api_key%]",
+                    "unit_of_measurement": "Unit of Measurement"
                 }
             }
         },
diff --git a/homeassistant/components/nightscout/translations/en.json b/homeassistant/components/nightscout/translations/en.json
index d8b4c441283ec7..0edba02099f29d 100644
--- a/homeassistant/components/nightscout/translations/en.json
+++ b/homeassistant/components/nightscout/translations/en.json
@@ -13,9 +13,10 @@
             "user": {
                 "data": {
                     "api_key": "API Key",
-                    "url": "URL"
+                    "url": "URL",
+                    "unit_of_measurement": "Unit of Measurement"
                 },
-                "description": "- URL: the address of your nightscout instance. I.e.: https://myhomeassistant.duckdns.org:5423\n- API Key (Optional): Only use if your instance is protected (auth_default_roles != readable).",
+                "description": "- URL: the address of your nightscout instance. e.g.: https://myhomeassistant.duckdns.org:5423\n- API Key (Optional): Only use if your instance is protected (auth_default_roles != readable).\n- Unit of Measurement: Choose between mg/dL or mmol/L.",
                 "title": "Enter your Nightscout server information."
             }
         }