Do you need a TEFL certificate to teach overseas?

Not necessarily, although certificates will generally make you a more competitive candidate for English teaching jobs, and some countries or institutes do have it as a prerequisite for teachers. However, other schools are sometimes willing to overlook the lack of a certificate if you have a teaching degree and/or previous experience, which it sounds like you do.

If you have a specific country or school in mind, see if you can find out what that particular location's requirements are about TEFL certification (is there a law about it, or is it on a case-by-case basis, etc).

Here's some more information on TEFL certification: https://www.gooverseas.com/blog/why-should-you-get-tefl-certified

it depends on the country. China and South Korea will want you to have a TEFL, whereas countries like Cambodia and Thailand may not care. Here's some more info I wrote about getting TEFL certified!
http://www.adventuresaroundasia.com/2015/08/27/should-i-get-a-tefl/

Great question, as mentioned, a TEFL Certificate isn't always listed a hiring requirement, however, schools always prefer teachers with more qualifications, credentials, and experience. A TEFL Certificate will make you stand out as a competitive applicant. In addition to all the knowledge you'll gain, you will also gain confidence and practical skills like lesson planning and classroom management. Do it for your future students! https://www.ciee.org/tefl