Footage showed Laxman Savadi apparently watching a clip on a phone and sharing it with his colleague, CC Patil.
Another minister, Krishna Palemar, the phone’s reported owner, also quit.
The pair denied watching pornography but said they were resigning to prevent embarrassment to the ruling Bharatiya Janata Party (BJP) government.
There was public outrage after news channels ran the story on Tuesday evening, and opposition parties demanded the resignation of the ministers.
“I have resigned to prevent any embarrassment to the party. All three of us will prove our innocence,” Mr Savadi said.
An inquiry has been ordered.
The footage of the ministers was captured by news channels positioned in the media galleries during a debate in the state assembly.
It shows Mr Savadi, the minister for co-operation, watching a clip on a mobile phone and then showing it to Mr Patil, the minister for women and child development.
Mr Patil told reporters: “We are not so uncivilised as to watch porn films.”
Mr Palemar, the state minister for ports and environment, said his innocence would be proved by the inquiry.