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