Moving money from eligible employer plans and IRAs into your TSP account is a great way for you to consolidate your retirement savings and take advantage of the TSP's very low costs.
You can have your traditional IRA or eligible employer plan send all or part of your money to the TSP. This is called a "transfer" or a "direct rollover." Use Form TSP-60, Request for a Transfer Into the TSP, for tax-deferred amounts. Use Form TSP-60-R, Request for a Roth Transfer Into the TSP, for Roth money.
You can receive money directly from your traditional IRA or plan and then later put it into your TSP account yourself. This is called a "rollover." You cannot roll over Roth money into the TSP and you must complete your rollover within 60 days from the date you receive your funds. Use Form TSP-60, Request for a Transfer Into the TSP.
The money that you transfer or roll over to the TSP does not count towards the IRC contribution limits so there is no restriction on the amount of money you can move.