D21 is a 80.1-kilometre-long (49.8 mi), north–south state road in Istria County, Croatia. A short section of D21 is a part of the European route E751. The northern terminus of the route is located at the Croatia–Slovenia border at the Dragonja River. There it connects to Koper, Slovenia, and Trieste, Italy, via the Slovene route G11 further north. D21 route is generally parallel to A9 motorway. The two form several junctions, either directly or via short connectors, at Buje, Bale and Vodnjan – towns served directly by D21. The southern terminus of the route is found in the city of Pula, at the southern tip of the Istrian Peninsula.