The Thrift Savings Plan (TSP) is a retirement savings plan for federal government employees and members of the military, offering similar benefits to a 401(k) in the private sector. Participants can contribute a portion of their pay on either a traditional (pre-tax) or Roth (after-tax) basis, and many receive matching contributions from their agency.

The TSP is known for its simplicity and low costs, offering a small selection of diversified funds with some of the lowest expense ratios available anywhere. For federal employees, it's typically one of the most important building blocks of a secure retirement.