The California-based 'I Marine Expeditionary Force' launched a command investigation after the picture was posted recently to a closed Facebook group for male Marines with more than 25,000 members, the Marine Corps Times reported.
The photo showed a corporal in uniform holding a rifle with his finger near the trigger. The words 'Coming to a gay bar near you!' appeared at the bottom of the photo.
The post follows Sunday's attack on a gay nightclub in Orlando, Florida, in which Afghan-origin gunman Omar Mateen massacred 49 people.
First Lt Thomas Gray, a spokesman for I MEF, was quoted as saying that the command has identified the Marine in the picture and the one who posted it on Facebook.
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"We cannot discuss details of an ongoing investigation, but I can tell you the command is taking this incident seriously," Gray said.
Marine officials have vowed to take "appropriate action" in response to the social media post, according to a statement.
"...This type of behaviour and mindset will not be allowed, and it is not consistent with the core values of honour, courage and commitment that are demonstrated by the vast majority of Marines on a daily basis," it said.
Michael Moss, the founder of the marine Facebook group, said on Facebook that the post was deleted as soon as it was reported and the person was banned from the group immediately.
"We do not tolerate hate speech," he said.
The incident coincides with a separate investigation by the FBI and San Diego police into an anonymous threat posted Tuesday on the men-seeking-men section of the Craigslist San Diego personal ads.