Baynes, who started 67 games at center for the Celtics last season, acknowledged the signing on Twitter, writing, "Staying where I wanted to be!! Last year added fuel to the [fire]. Bleed green banner 18" with a series of green shamrocks emojis.
The Celtics were hoping to retain Baynes, a bruising big man who helped Boston post the best defensive rating in the league last season. Baynes owned the best individual defensive rating in the NBA among qualifiers.
Baynes played just 18.3 minutes per game last season but often started alongside Al Horford. A healthy Boston roster will likely shuffle Baynes to a reserve role in the 2018-19 season, but part of the reason he endeared himself in Boston is the way he embraced any role.
Celtics coach Brad Stevens told ESPN this week that Baynes "did a world of good for us" while detailing what he brings to the team.
"Just his contagiousness. He has, in a lot of ways, just a spirit about him that you want your team to embrace," Stevens said. "His work ethic, his everyday mentality. He brings just a juice every single day. And he's an elite defendant. ... He's a really good player, and he's a really tough guy."
The Celtics drafted big man Robert Williams with the 27th pick in last month's draft, adding depth to their frontcourt.
Boston's focus now shifts to restricted free agent Marcus Smart, another key part of last season's team that surged to the cusp of the NBA Finals. Celtics brass have repeatedly said they desire to bring Smart back, and Boston can match any offer sheet Smart signs this summer.