What are the tax implications (if any) if I, a US citizen receive a lump sum of cash as a gift from an overseas relative?

There are no tax implications; since the giver is a non-resident alien, no gift taxes are due.

If the amount exceeds $100,000, Form 3520 has to be submitted to report the source of the funds, but that is information only.

Thank you so much.

Glad to help!