Citadel has emerged as a top dealer in US interest rate swaps, becoming one of the first non-bank firms to step into a breach created by postcrisis rules overhauling trading in those derivatives.

The Chicago hedge fund firm's Citadel Securities unit has the largest market share by number of trades and the third largest by dollar volume in the second quarter, according to documents reviewed by The Wall Street Journal showing the firm's rankings on a swaps platform operated by Bloomberg. As of the first quarter, the firm said it was number three by number of trades and number four by dollar volume. The platform covers nearly half of all customer trades in the derivatives, according to swaps data tracker Clarus Financial Technology.