I am a self-employed US citizen residing in Portugal, which has a totalization agreement with the US. Are my SS payments in Portugal deductible from my US SE liability?

If you are self-employed and working in a country with a Totalization Agreement. you do not have to pay self-employment taxes/ 

Thank you, this is what I thought! However, do you know if there's any way to include this information when doing my taxes through TurboTax? If not, where do I include that information on the 1040, or is it a separate form? I'm really having a hard time finding this information.

This can be done in the self-employment topic.  Here is a link with instructions.  Be advised you will need to print and mail your return with a copy of the certificate of coverage as outlined in the link.