The file named
footer.php contains the copyright info:
Designed by ...
Is it OK to remove theme credits from footer?
Is it legal?
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On a free and publicly available theme, yes, you can!
If you built the theme yourself, yes, you can!
If the theme is under a GPL license or completely public domain, yes, you can!
If you're a client of a developer who has built the theme for you however it may not be so simple. It may be in your contract that it be put there, and you should have negotiated this when procuring a developer or agency. Consult your designer/developer or contact a lawyer.
Buying a theme from a theme shop is also questionable, check their terms of service and the agreement, and if in doubt contact a lawyer
If a premium theme has a free lite version then it may also be questionable, ask the original author and check the license, and if in doubt contact a lawyer
Keep in mind this is not a forum for legal advice, and we are not legal professionals. We know some cases it is indeed legal, but for other cases we cannot provide solid legal advice. What is legal also changes from state to state.