401k and TSP deferrals

Can an employee who is actively contributing to a 401k Plan also contribute to a Thrift Savings Plan offered by Military at the same time?

Yes, but they share the same 402(g) limit.

The TSP is offered to federal employees, including uniformed service members.  It is subject to the 402(g) limit.  In 2019, the 402(g) limit is $19,000 annual elective deferrals.

So somebody who is eligible for federal TSP and also eligible for an employer 401(k) plan can contribute to both.  But the total elective deferrals for the year, to all plans, cannot exceed $19,000 in 2019.

However, the annual additions limit is separate if the employers are unrelated.  So if the 401(k) plan sponsor is not the federal government but a separate employer, then they each have their own annual additions limit.  Annual additions include elective deferrals, non-Roth after-tax contributions, employer nonelective contributions and other allocations.  In 2019, the annual additions limit is $56,000.

That means the individual could defer at most $19,000 pre-tax or Roth deferrals in 2019 into one or both plans, but can make non-Roth after-tax contributions up to the annual additions limit - after accounting for employer nonelective contributions and other allocations.

Thank you!