Retirement Security Act of 2017 This bill amends the Internal Revenue Code and the Employee Retirement Income Security Act of 1974 (ERISA) to modify various requirements for employer-provided retirement plans. The bill modifies the qualification requirements for certain multiple employer plans with pooled providers. The bill applies to defined contribution plans that: (1) are sponsored by employers all of which have both a common interest other than having adopted the plan and control of the plan, or (2) have a pooled plan provider. Such a plan that meets specified requirements may not be disqualified or otherwise lose its tax-favored status because a participating employer fails to take actions required with respect to the plan. The bill also: (1) permits pooled employer plans that meet certain requirements to be treated under ERISA as a single employee pension benefit plan or single pension plan that is a multiple employer plan, and (2) modifies reporting requirements for pooled employer and multiple employer plans. With respect to 401(k) retirement plans, the bill: (1) modifies requirements related to default rates for elective deferrals under automatic enrollment plans, the election of safe harbor 401(k) status, and nondiscrimination rules; (2) allows a business-related safe harbor adoption tax credit for small employers, and (3) requires the Department of the Treasury to simplify regulations regarding the timing of participant notices. The bill also: (1) increases the limit on the amount of the credit for small employer pension plan startup costs, (2) allows a business-related tax credit for small employers who include and maintain an automatic contribution arrangement in an employer-sponsored retirement plan, and (3) requires Treasury to modify tax forms to permit individuals to claim the saver's credit on Form 1040EZ.