Huang Xingguo, 62, was suspected of "serious violation of disciplines", South China Morning Post quoted the Central Commission for Discipline Inspection as saying.
Huang, who was made mayor of Tianjin in 2007 and took on the additional role of acting party secretary in 2014, had not yet been permanently appointed.
Before falling under suspicion, Huang was seen as a contender for the party's 25-member Politburo, possibly joining the decision-making body at next year's 19th party congress.
The investigation was announced 13 months after a deadly warehouse blast in Tianjin, for which Huang previously said he bore "unexcusable" responsibility.
The blast killed 162 people and displaced thousands of residents, most of whom were living less than one kilometre from the chemical storage site and saw their apartments destroyed. Many residents blamed lax regulations for the explosion.
