The victim, in his late 60s, removed his face covering to talk to his mother, who is in her 90s and hard of hearing.
As he did so the suspect came up behind him, demanded that he put his mask back on, then punched the victim in the face. He was left with bruising around the eye.
The incident happened on a Blackpool tram at about 8.30pm on September 4.
PC Laura Alves, of Lancashire Police, said: “This was a shocking attack on a man who was simply trying to speak to his mother, who is hard of hearing.
“We urgently need to speak to the man in the CCTV still and I would ask that anyone who recognises him to contact police.
“Similarly, I would appeal to the man directly to contact us if you see this appeal.”
The attacker is described as in his 30s, about 5ft 6in tall, of large build, wearing all black with a grey face mask; he had a shaved head with dark hair and was wearing glasses.
Anyone with information can call police on 101, quoting log 0428 of September 5, 2020. Alternatively, call independent charity Crimestoppers, in confidence, on 0800 555111.