Nelsen joins Spurs on a free transfer after Blackburn agreed to the termination of his contract.
Nelsen will be in Spurs’ squad for Monday’s Premier League game at Liverpool.
“He is fit and eligible to play on Monday,” Tottenham’s assistant manager Kevin Bond said.
“I’ve just had a cup of tea with him in the canteen. He is looking forward to the challenge.”
Lyle Yorks, Nelsen’s US-based agent, had told the Washington Post on Wednesday: “It’s for six months, with a one-year option. It all unfolded quickly.”
“[Spurs manager] Harry Redknapp has always been a big fan of Ryan.
“When the opportunity came up, it really played itself out.”
