Witsel had been chased by Juventus since the previous transfer window, according to Italian media reports.
He was quoted by Italian media on Monday saying that his decision to move to China rather than Turin had been a "really difficult" choice.
"On the one hand, there was a great team and a top club such as Juventus, but on the other, there was an offer which I could not refuse for the sake of my family," he said.
It has been widely reported that Witsel, who has won 80 caps for Belgium and played at the 2014 World Cup and Euro 2016, could earn 18 million euros a year at his new club.
Last week, former Manchester United, Manchester City and Juventus forward Carlos Tevez joined Shanghai Shenhua from Boca Juniors in his native Argentina in another big-money move which saw the Chinese club reportedly pay 84 million euros ($87.65 million).
In another move, Brazilian Oscar joined Shanghai SIPG from English Premier League side Chelsea, while other recent arrivals in China have included Italy striker Graziano Pelle, Argentine Ezequiel Lavezzi and Colombia forward Jackson Martinez.
Witsel will be coached at Tianjin by Italian World Cup winning defender Fabio Cannavaro.
A gifted all-round midfielder with an aggressive streak, Witsel was raised at Standard Liege before moving to Benfica in 2011. He spent one season with the Portuguese side and then joined Zenit where he played 180 matches and scored 22 goals.
(Writing by Brian Homewood in Berne; Editing by Toby Davis)
