Maardu is a town and a municipality in Harju County, Estonia. It is part of Tallinn metropolitan area. The town covers an area of 22.76 km² and has a population of 15,332 (as of 1 January 2019).

The Port of Muuga, the largest cargo port in Estonia, is partly located in Maardu.

According to the 2000 Census, the population was 16,738. 61.7% were Russians, 19.9% Estonians, 6.6% Ukrainians, 5.7% Belarusians, 1.5% Tatars, 0.9% Finns, 0.6% Poles, 0.5% Lithuanians, 0.2% Latvians, 0.2% Germans and 0.1% Jews and 1 Cuban. The proportion of Estonians was one of the lowest (if not the lowest) in Central and Western Estonia.

Outside the town (in Maardu village), south of the road to Narva lies Maardu manor, one of the oldest preserved baroque manor houses in Estonia. It traces its origins to 1389, but the current building dates from the 1660s with additions made in the 19th century. The landlord of the manor Herman Jensen Bohn in 1739 funded the printing of the first bible printed in Estonian.