Nelsen's contract with Rovers had been cancelled, which made him a free agent and able to sign for Harry Redknapp's side. The deal has now been confirmed by the 34-year-old's agent Lyle Yorks.
"We settled with Blackburn and have reached agreement with Spurs," Yorks told the Washington Post
"It all unfolded quickly. Harry Redknapp has always been a big fan of Ryan. When the opportunity came up, it really played itself out today.
"We're just awaiting clearance. It's for six months, with a one-year option."
Nelsen joined Blackburn in 2005 from American side DC United.