Spencer's agent, Jordan Woy, met with Cowboys officials late last week about a possible new deal, but nothing was finalized.
With Spencer now signed, he's expected to attend the team's voluntary offseason conditioning program, which started its second week on Monday.
The Cowboys placed the franchise tag on Spencer because they lacked other options at outside linebacker.
While Spencer has never had more than six sacks in a single season, team owner and general manager Jerry Jones has hinted that schematic changes in the defense could help Spencer become a better player.
ESPNDallas.com's Todd Archer contributed to this report.