The veteran left tackle must sit out the next three games, however, because he and wide receiver Vincent Jackson were placed on the roster exempt list by general manager A.J. Smith on Aug. 20. McNeill will be eligible to play in Week 6 at St. Louis.
The two sides have the next few weeks to try to work out a long-term deal.
McNeill had been holding out in search of a long-term deal with the team.
Although the Chargers had been in talks to trade Jackson, it had not entertained offers for McNeill and San Diego had moved forward without the two-time Pro Bowler.
McNeill said on his Twitter account, "great to be back today.. GO BOLTS." He signed in time to travel with the Chargers to Seattle for their game on Sunday.
McNeill's original four-year contract expired at the end of last season. But because this is an uncapped year, he would have needed six seasons to become an unrestricted free agent.
When McNeill hadn't signed his $3,168,000 tender as a restricted free agent by June 15, the Chargers were entitled to reduce the tender to 110 percent of his 2009 salary, or $630,000. That will be further reduced by each game he doesn't play.
Adam Schefter is ESPN's NFL Insider. Information from The Associated Press was used in this report.