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He had been looking for investors for the last 12 months after previous majority shareholder Anton Zingarevich failed to complete a full takeover.
The Thai consortium have agreed to keep Madejski, who has been with the club since 1990, on the board.
Asked whether he felt the new owners had saved the club, he said: "In many ways, I'd say they have."
Speaking to BBC South Today, he continued: "There were other parties interested, but these are the ones I favoured because when you're choosing new owners of a treasured club like Reading you have to be incredibly careful.
"We've had some colourful suitors in the past, but these people are decent, straightforward people, and I'm grateful for that."
The amount of money involved in the takeover has not yet been disclosed.
But, under the agreement, businesswoman Khunying Sasima Srivikorn, now also co-chairman, and Khun Sumrith Thanakarnjanasuth have each taken 25% of the club, while the other 50% of the shareholding has been bought by Khun Narin Niruttinanon.
Their takeover ended months of speculation regarding potential investors into the Championship club.
"I've had lots of sleepless nights thinking about Reading Football Club over the years, but I think the last 12 months have been the most challenging," Madejski said.
"I'm just delighted that at long last we've reached a situation we can look positively to the future."
However, the 73-year-old has warned the consortium against making sweeping changes too soon after the completion of the deal.
"Like the old saying, if it ain't broke why fix it?," he added.
"They're obviously ambitious, they want to get Reading into the Premier League as soon as they can, so they will be making sure they make a positive input.
"We welcome that because change is always a good thing and a fresh pair of eyes can see things perhaps we've missed."
Reading co-chairman Sir John Madejski says the club's new Thai owners may have "effectively saved" the club.