The IBM Simon went on sale to the public on 16 August 1994 and combined mobile phone technology with a wide range of computing features.
To mark the 20th anniversary, London's Science Museum is putting it on display in its new Information Age gallery.
"The Simon wasn't called a smartphone back then," said curator Charlotte Connelly.
"But it had a lot of the features we see today. It had a calendar, it could take notes and send emails and messages and combined all of this with a cell phone."
It only had one hour's battery life - so it wasn't very successful”
Weighing in at 500g (1.1lb), the Simon was not exactly pocket-sized. However, Ms Connelly insisted the design was ahead of its time.
"It looks like a grey block but it's not as big as you'd imagine," she said. "It had a stylus and a green LCD screen, which is similar in size to the iPhone 4. In fact, it's not a bad looking thing."
IBM's pioneering product was also the first mobile phone to feature software apps and could be linked up to a fax machine.