Sabbir Miah. 29, of east London, posted footage glorifying fighters with the terror group to hundreds of people on a private group on June 14 last year.
Jurors were told how he had five previous convictions for similar offences committed in 2016 and 2017. He had been jailed for three years and four months.
Prosecutor Diana Wilson had previously told the trial: “The very same day Mr Miah joined the group he posted a video which could be viewed by anyone in the private group.
“The video includes a black flag that has been adopted by Islamic State and is produced by Al-Hayat media, who produce IS propaganda.
“It clearly is an IS publication which glorifies IS fighters and is intended to do so.”
Following his arrest at a temporary address in London on November 2 last year, Miah said: “It’s weird because I haven’t posted anything.”
Ms Wilson told how Miah had three convictions for distributing a terrorist publication relating to IS videos on an earlier Facebook account in 2016.
In early 2017, he posted further IS videos on Instagram and WhatsApp resulting in two more convictions.
The court heard he had pleaded guilty to those offences. He is due to be sentenced on October for the latest offence.
Speaking after the verdict, Commander Richard Smith, who leads the Metropolitan Police’s counter terrorism command, said: “Miah was clearly determined to seek out and share this extremist content online again, even in the knowledge that it could result in a further terrorism conviction.
“Terrorist propaganda is extremely damaging to those viewing it and to society as a whole, and we work hard with our partners to identify and arrest people like Miah, and to get this kind of harmful content removed from the internet.”