The other four are Manchester, Sheffield, inner London and south Wales.
The report says the sector employs 32,000 people in Belfast.
The top three specialist sectors were games development, publishing and software development, and data management and analytics.
Stephen McKeown, CEO of Analytics Engines, said the quality of the workforce in Belfast was attracting companies.
“There’s a lot of people here who are very motivated, who are well-educated and understand the digital sector,” he said.
“Plus it’s a small place. I think having that kind of community feel to it makes a very big difference and it’s something you don’t find elsewhere.”