Retirement Security Act of 2015

Directs the Secretary of the Treasury to: (1) prescribe final regulations to permit employers to participate in multiple employer pension benefit plans, (2) promulgate regulations or other guidance to simplify and clarify rules relating to the timing of participant notices required under tax-preferred pension plans and the automatic escalation rules, and (3) modify the 1040EZ tax return form to allow taxpayers to claim the tax credit for retirement savings (saver's credit) on such form.

Amends the Employee Retirement Income Security Act of 1974 (ERISA) and the Internal Revenue Code, with respect to employer pension benefit plans, to: (1) allow employers to maintain a tax-exempt multiple employer pension benefit plan even if the employers sponsoring the plan share no common interest, (2) modify requirements for secure deferral arrangements with respect to nondiscrimination and employer matching contributions, and (3) allow employers with not more than 100 employees a business-related tax credit to cover increased matching contributions required by this Act.