The extra 800 tickets are now on sale for the clash with the Springboks on August 21.
Connacht Rugby Commercial Manager Fiona Keys is delighted that capacity can be increased for the occasion.
"The interest shown by fans has been phenomenal and we're delighted that we can accommodate so many more fans than we originally thought," she said.
"The only previous times we've brought in temporary seating were for the two European Challenge Cup semi-finals a few years back, so it'll be great to have such a big crowd here for a pre-season game."
She added that tickets would be sold on a first-come basis and that space had been reserved to facilitate new season ticket holders.
The additional tickets which have gone on sale will cost EUR30 each and are available online at www.ticketmaster.ie or from the Connacht Branch offices on College Road.
Season tickets have been selling well for what promises to be an exciting campaign for Connacht Rugby and it has been decided to hold the last few tickets for the South Africa match for new season ticket holders.
Fiona Keys stresses that these remaining places will be on a first-come basis.
"There has been a huge response to the South Africa match and to the season ticket campaign.
"The fact that a season ticket also includes the pre-season games against Castres on August 28 and September 1 is an added bonus and that is before the Magners League and European Challenge Cup kicks-off," she explained.
Connacht Rugby's season tickets are available on line on www.ticketmaster.ie and are also available through the Branch offices at the Sportsground. The season tickets will be sent out in the first week of August.