Generally, an RRSP contribution is not deductible on a US tax return.
There is, however, an exception under the Canada-US tax treaty that allows a RRSP deduction in certain situations. In particular, if the RRSP contribution is made via employee contributions to an employer sponsored group RRSP plan, then the contribution is deductible on the US tax return.
But there is a limit on how much you can contribute. Specifically, the RRSP contribution is limited to the lower of your RRSP deduction limit in Canada or your 401(K) limit (currently at $18,000 for those under the age of 50, $24,000 for those 50 and over.)
In addition, you will need to notify the IRS that you are lowering your taxable wages by the RRSP contribution. You do this by filing a Form 8833 with your US return and claiming an exemption under the tax treaty.