Neagle returns for a fourth stint with his hometown team, having played for the Sounders in 2009, 2011 and from 2013 to 2015.
"We are very pleased to welcome Lamar back to Seattle as we hit the home stretch of the 2017 season," Sounders general manager Garth Lagerwey said in a news release. "He's a good, veteran player with an extensive history with our club, and he gives us another solid option in the attack going forward."
Neagle made 54 MLS appearances (30 starts) over a season and a half with United. The 30-year-old was the club's leading scorer in 2016, with 10 goals in the regular season and playoffs, but had found the net just once this year.
Neagle, who also has played for the USL's Charleston Battery, Finnish side Mariehamn and the Montreal Impact, has recorded 40 goals and 25 assists over his MLS career.
The move clears salary cap space for last-place United while adding depth on the flanks and up top for the MLS Cup champion Sounders, who sit fourth in the Western Conference at 9-7-7.