Kochman, who was named global co-head of M&A only in March, left for Citigroup with colleagues Jim Glerum and Justin Friesen. Friesen was deputy head of US M&A at UBS, while Glerum headed diversified industrial banking.
London-based global co-head Liam Beere, who was promoted at the same time as Kochman, continues with the bank, UBS has said.
Kochman, Glerum and Friesen, all based in Chicago, are joining former colleagues who have moved from UBS to Citi recently, including Kevin Cox and Francis Tucci.
Hong Kong-based Jason Wortendyke, head of UBS Asian industrials, also left the bank, a spokeswoman confirmed. It was not clear where Wortendyke was headed.
UBS has seen scores of investment bankers leave in the last three years as the financial crisis hit the Swiss bank hard and forced it to put unpopular curbs on compensation.
The bank fell to No. 13 in the first-quarter rankings of advisers involved in US deals, down from No. 8 in the same period last year, according to Thomson Reuters data.
In March, the bank named new heads of investment banking across the globe as it tried to stanch the talent drain, installing Aryeh Bourkoff, a well-known media banker, as head of Americas investment banking.
Bourkoff has hired bankers as he tries to build out the investment bank, but UBS continues to bleed talent.
Kochman, who joined UBS in 2004, was promoted to global co-head of M&A at the same time as Bourkoff. He will become head of North America M&A at Citi based in Chicago.
Glerum will become chairman of Citi's regional banking for North America. UBS announced earlier on Wednesday that Glerum and a few other bankers had left the firm.