TSP Modernization Act of 2017 This bill modifies the rules relating to withdrawals from the Thrift Saving Plan (TSP) accounts of former federal employees and Members of Congress. Under current law, such employees and Members may make only one partial withdrawal upon reaching age 59-1/2 while employed or one withdrawal after retirement. The bill permits an unlimited number of such withdrawals. The bill also eliminates: (1) the withdrawal election deadline (i.e., by April 1 of the year after the year in which the plan participant turns 70-1/2 or is separated from service); and (2) the limitation on age-based in-service withdrawals.