The news ends four months of uncertainty about whether Motorola would actually join the venture, which is called Symbian. The US group announced its intention to join when the new company was formed in June. However, industry sources said Microsoft, the software giant, had put intense pressure on Motorola not to join the venture.
Earlier this month a leaked memo from Bill Gates, Microsoft's founder, identified Psion as one of the main threats to the US company's future.
Symbian was set up to develop software based on EPOC, the operating system invented by Psion, to be used in a new generation of portable computing devices capable of being connected to each other through mobile phone networks.
"We are delighted to join Symbian. It is a very positive move for us," a spokesman for Motorola said.
The US group will invest pounds 28.75m in the new venture in return for a 23.1 per cent shareholding - equal to the stakes held by Nokia and Ericsson. Psion's shareholding in Symbian will be diluted from 40 per cent to 30.7 per cent.
Motorola will also contribute its Starfish technology, which allows computers to synchronise with each other, to Symbian.
The venture intends to licence the software to as many companies as possible in an attempt to develop an industry standard for portable computers.
A spokesman said discussions with potential licensees were continuing. He would not rule out other companies taking equity stakes in Symbian.
Shares in Psion jumped 46p to 485p on the news as investors woke up to the huge potential of the joint venture.
Meanwhile, Psion announced that Nicholas Myers, its former managing director who is taking over as chief executive of Symbian, was resigning as a director of Psion.