The possible move was first reported here on www.reading-mad.co.uk
The deal should be completed tomorrow in time for the former Crystal Palace and Nottingham Forest loanee to make his debut against Rotherham on Saturday.
He will wear the number twenty-nine shirt, which was worn by John Salako last season.
The move for Upson should to pave the way for Adi Viveash to leave the club. Last week Viveash wastold he could leave on a free transfer once a new centre-back was signed.
Upson, who has suffered from long spells on the sidelines in recent years because of injuries, was signed by Arsene Wenger for a fee of £2 million from Luton Town in 1997.
Speaking to readingfc.co.uk Upson said: "I met the lads before training, which was good. I am very pleased to be at Reading, I've noticed that it's a good club.
"I'm looking forward to playing her because the club's got a good foundation."
The move was first reported on Reading Mad, exclusively, on Monday before several national news organisations picked up on the news, however, there was confusion as to whether the deal would go through, initial reports indicated that Upson wasn't keen on the switch.
He explained: "I knew about the deal this week, but there was a lot of confusion about the transfer window. I was pleased to have the opportunity to finally come here and I am looking forward to playing some games.
"I already know James Harper from when he was a trainee at Arsenal, so that helps. I have no problem settling in at different clubs, you take it all in your stride."
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