Lively Rotariu put on a sizzling show of dribbling before giving Sweden keeper Karl-Johan Johnsson no chance with a brilliant left-foot shot in the 57th minute in the southern city of Craiova.
Sweden, who knocked out Italy in a two-legged playoff to book their ticket for the tournament in Russia this year, enjoyed lots of possession but failed to impress, offering little in attack.
The Swedes, who suffered another setback on Saturday when they were beaten 2-1 by Chile at home in another warmup, have been drawn with defending champions Germany, Mexico and South Korea in World Cup Group F.
Romania failed to qualify for the finals for a fifth consecutive time.
The match at the Ion Oblemenko Stadium was interrupted for a few minutes at the end of the first half after a number of fireworks were thrown by Romania fans.
(Reporting by Angel Krasimirov; Editing by Toby Davis)
