Ekwah was still waiting for news of whether his loan move from Saint-Etienne had gone through after Monday's 7pm deadline, with an EFL deal sheet submitted to prove that an agreement had been reached in time.
His move, which includes a permanent option, was officially confirmed on Tuesday morning and sees him join a midfield group featuring Papy Mendy, Imran Louza, Edo Kayembe.
Hector Kyprianou is out with an ankle injury, while Edoardo Bove is building fitness after signing in January.
Ekwah describes himself as an all-round midfield player and outlined what convinced him to sign for the Hornets, after a dispute with the French club meant he did not play in the first half of the season.
He told the club site: "I can play in midfield as a six or an eight.
"I'm left-footed and like to get on the ball, even with pressure.
"I like to pass the ball around and I'm a player that can fight to get the ball back as well as being creative in midfield.
"I'm joining such a good club that has a really big objective to get to the Premier League. I'm pleased to join them and help them get there."
Former Sunderland midfielder Pierre Ekwah has joined WatfordFormer Sunderland midfielder Pierre Ekwah has joined Watford (Image: Reuters)
Ekwah remembers featuring at Vicarage Road for Sunderland, where he impressed at Championship level during the 2023/234 season.
He said: "It's a good ground. I remember the stadium and the pitch were really nice.
"I'm really excited to make it my new home. When we went there we lost, but now I'm wearing a different shirt, so hopefully I can make new memories."