The Retirement Investment in Small Employers Act aims to significantly boost incentives for very small businesses to offer retirement plans to their employees. It amends the Internal Revenue Code to provide an enhanced startup tax credit specifically for "qualified microemployers." Under this bill, qualified microemployers would receive a 100 percent tax credit for eligible retirement plan startup costs, a substantial increase from the current 50 percent. The maximum credit amount for these employers would also rise from $500 to $2,500 . A "qualified microemployer" is defined as an employer with 10 or fewer employees that establishes or maintains an eligible employer plan, provided the plan accepts a specific type of matching contribution. These changes are intended to encourage more micro-businesses to establish retirement savings options and would apply to taxable years beginning after December 31, 2024.