WASHINGTON, April 14 (Reuters) - Bart Chilton, an outspoken former regulator who spent more than six years as a member of the U.S. Commodity Futures Trading Commission, will join the law firm DLA Piper, the company said on Monday.

Chilton, a Democrat who left the CFTC last month, witnessed its transition from a little-watched overseer of futures markets into a powerful body put in charge of the $690 trillion swaps market after the credit crisis.

"Bart will be an invaluable resource for clients who will be subject to increased regulation over the next few years," the firm said in a statement.

Chilton, currently working on a book called "Theft" about how Wall Street and Washington intersect, will work in the swaps practice of DLA Piper, which has grown into one of the largest law firms in the world in the past decade.

He will also work on "thorny issues" related to how the U.S. rules for derivatives apply abroad.

In his speeches as a regulator, Chilton frequently spoke out against the influence of speculators and high-frequency traders on commodity markets, defending the interests of farmers and others who use derivatives to hedge price risks.

Often an outspoken critic of large banks, Chilton had urged the U.S. Federal Reserve to reverse its policy of allowing investment banks' widespread activities in commodities, saying it created a conflict of interest.

Brewing companies last year complained before Congress that banks were pushing up the price of aluminum, which is used to make beer cans, by delaying delivery times and creating long queues for the metals warehouses the banks own.

(Reporting by Douwe Miedema; Editing by Leslie Adler)