No, it is not illegal for a free US citizen. However, prisons specifically prohibit gang membership because prisoner are not free citizens. Many gang members in prison try to conceal their membership and security guards refer to gangs as “security threat groups” as a way of denying legitimacy.
The KKK is legal. Prison gangs are legal. There nothing banning the existence of a gang as that apparently impedes first amendment rights.
What is illegal is the various things these gangs do.
“Gang affiliation is not prohibited in the US; however some states have imposed prohibitions especially when it comes to youth gangs.”
https://www.legalmatch.com/law-library/article/gang-affiliation-laws.html
No, it is not illegal for a free US citizen. However, prisons specifically prohibit gang membership because prisoner are not free citizens. Many gang members in prison try to conceal their membership and security guards refer to gangs as “security threat groups” as a way of denying legitimacy.
Source: Correction: Gangs and Violence in Prisons - SAGE edge