A prisoner told police he woke in the night to find his cellmate hanging from a heating pipe.
Simon Wishart, 22, vividly described to detectives how he saw Gareth Russell's legs swinging off the ground.
Wishart, formerly from Eastbourne, claimed he immediately jumped out of his bunk and released Russell, whose body dropped to the floor with a "thud."
He is on trial accused of murdering Mr Russell in cell 19 in the care and support unit for vulnerable prisoners at Lewes prison.
Wishart is accused of strangling the dead man on the night of April 24 last year before claiming Russell had committed suicide.
The jury has been told that after Russell died, Wishart bragged about killing his cell mate.
He is said to have confessed he throttled Mr Russell while singing the American national anthem.
In an interview with police Wishart described finding his cellmate hanging from a ligature made from torn bedsheets.
He said he hammered on the cell door to alert a jailer who he told: "Get me out of here. My cell mate has hung himself."
Wishart told detectives he and Russell had been friends during the three months they had been cellmates.
He said: "We got on well. There had been no fall outs with him."
Russell, 25, had been convicted of manslaughter days before his death and was beginning a six-year sentence.
Wishart denies murder. The jury have been told they have to decide whether the killing was a cold-blooded murder or manslaughter due to diminished responsibility.
The trial continues.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article