The acquisition, which will see Citigroup pay $106.2 million in cash along with 11.17 million in new shares, is aimed at enabling the bank to capitalize on the move towards electronic trading.
Citigroup, which plans to expand the platform to have a global reach, announced that it expects to double its share of volume on electronically traded markets through the acquistion.
A statement from James Forese, Citigroup's head of global equities, said: "ATD's strong management team and the company's market-leading technology will help us provide best execution to all of our clients by enhancing liquidity through the aggregation of greater flow.
"The combination of Citi's Global Equities business and ATD will create a leading US stock trading platform."
The deal for ATD follows other recent acquisitions from Citigroup, including Knight Capital Group and Lava Trading as it attempts to broaden its customer base.