Commissioner Robert R. Davis of the Commodity Futures Trading Commission resigned Monday to become senior vice president for planning and development at the New York Mercantile Exchange, R. Patrick Thompson, Nymex president, said.

The appointment is effective May 21.''He brings to our exchange broad-based expertise in international finance, strategic planning and negotiating and economic research and policy development, as well as an understanding of the current regulatory environment," Mr. Thompson said.

In his letter of resignation to President Bush, effective Monday, Mr. Davis called the CTFC "a model of good government (that) constructively channeled the 'invisible hand' of the market and avoided laying on the 'dead hand' of excessive regulation."

An economist, Mr. Davis was chairman of the CFTC's financial product advisory committee. He was nominated to the CFTC by President Reagan and confirmed by the Senate in 1984.

Mr. Davis fills the Nymex post held by Jan B. Kay, who resigned two weeks ago to enter private consulting.

Mr. Davis had been mentioned last year as a candidate for several top staff positions when Nymex launched its management reorganization following the July resignation of Rosemary T. McFadden, Nymex president, and the appointment of Mr. Thompson.

''He will be very productive to Nymex," an exchange official said, calling Mr. Davis "A free-market individual" and "a good thinker."

Mr. Davis' financial and economic background are expected to be tested as the exchange develops its course for the coming decades.

At the CFTC, he initiated revisions of CFTC regulations on hedging and risk management as well as analysis and review of off- exchange derivative instruments such as hybrid debt, equities, securities and swaps.

Prior to joining the CFTC, Mr. Davis served as senior economist of the Congressional Joint Economic Committee. He was vice president and economist with Harris Trust and Savings Bank in Chicago, and was a financial economist with the Federal Deposit Insurance Corp.