The Designated Player is expected to join training on Wednesday ahead of D.C. United's home match against Austin FC on Saturday (7:30 pm ET | MLS LIVE on ESPN+).
The Greek international initially signed a pre-contract agreement to join the MLS club as a free agent this summer when his contract with Austria's Rapid Vienna expired, but D.C. and Rapid later reached an agreement over compensation for Fountas to join immediately rather than in the summer. The Washington Post reported that the transfer fee was around $400,000.
Fountas, 26, has made eight appearances for Greece's national team and has scored 64 goals and recorded 37 assists across 255 professional appearances. His club career began at Greece's AEK Athens before a move to Austria's Red Bull Salzburg, building toward his starring role at Rapid Vienna.
The attacker has been best deployed as a second striker, playing underneath a traditional forward. That would suit Fountas well in manager Hernan Losada's 3-4-2-1 formation, though he can play elsewhere in the attack.
Fountas isn't the only key attacker to join the club this transfer window, following Ecuador international center forward Michael Estrada on loan from Liga MX's Toluca.
Estrada, also 26, has two goals in three appearances but hasn't played in a month. He was away from the team for family reasons and then wasn't in the squad following international duty in the March window, helping Ecuador qualify for the World Cup. The Black-and-Red did not have a game last weekend.
Elsewhere, midfielder Chris Durkin, re-acquired by D.C. United at the end of March from Belgium's Sint-Truidense V.V., started in a 1-0 loss to Atlanta United in his return to the club that signed him to his first professional contract.
D.C. United have taken six points from their first five games as they work towards finding their best XI in Losada's second year, with Durkin re-assimilating into the team, Estrada getting settled and Fountas preparing for his debut.