Spikes' agent, Terry Watson, said Monday night it was a four-year, $3.21 million deal.
Spikes was en route to Gillette Stadium to sign the deal, which includes a $960,000 signing bonus and a one-time incentive of $450,000, according to Watson.
The agreement was first reported by the Boston Herald.
"He's refreshed and pleased that the contract is done," said Watson. "He cannot wait to get to the facility. He loves to play football and is ready to go about what he loves."
Spikes, the final selection of the second round, will vie for playing time at inside linebacker next to 2008 Associated Press Defensive Rookie of the Year Jerod Mayo. He will be competing with Gary Guyton (third year) and Tyrone McKenzie (second year).
With Spikes in the fold, the Patriots' lone unsigned pick is first-rounder Devin McCourty.
Mike Reiss covers the Patriots for ESPNBoston.com. Follow him on Twitter.