The actor released a statement through his publicist on Tuesday night, stating that Bellamar's allegations are false.
"I unequivocally deny the appalling allegations being peddled about me. It did not happen," the statement read. "It takes a great deal of courage for victims to come forward with their histories, and my hope is that the allegations about me that didn’t happen, do not detract from stories that should be heard."
"'Member grabbing my boobies on the [sofa] without asking?? 'Member when I tried to leave; you grabbed me by the a--, looked at yourself in the mirror, & said what a 'beautiful couple' we made?" she tweeted.
She later added: "I was led to @jeremypiven's trailer by a young, blonde woman w/a clipboard (I presumed a PA) to discuss my 'career'. My 'potential'."
CBS, network home of actor Piven's series "Wisdom of the Crowd," announced in a statement on Tuesday that it will investigate Bellamar's accusations.
"We are aware of the media reports and are looking into the matter," read Tuesday's statement from the network.
HBO also released a statement to The Hollywood Reporter on Tuesday, in which the network stated that employees only learned of Bellamar's allegations through press reports.
"Everyone at HBO and our productions is aware that zero tolerance for sexual harassment is our policy," the statement read. "Anyone experiencing an unsafe working environment has several avenues for making complaints that we take very seriously."