Terms of the investment are not being disclosed, although unnamed sources quoted by LBO wire suggest the deal values Sterigenics at more than $2bn (€1.8bn).
The transaction is expected to close in the third quarter this year.
Sterigenics provides sterilization and ionization services for the medical device, pharmaceutical, food safety and specialty materials industries. GTCR paid former owner Silverfleet Capital $675m for the firm in 2011.
In the years since, GTCR has backed several acquisitions by Sterigenics, including Florida-based Food Technology Service, Canada’s Norton and Italian gamma ray sterilisation firm Gamma and in November.
Rumours last year suggested GTCR was looking to offload Sterigenics.
Reuters reported that the investor had hired Jefferies and Goldman Sachs to seek a buyer, valuing the contractor at $1.5bn.
News of Warburg Pincus’ investment comes weeks after Sterigenics’ rival Steris announced it had rescheduled a shareholder vote on whether to buy UK-based contract sterilisation services firm Synergy Health.
Steris announced that the US Federal Trade Commission (FTC) had asked it for more information about the deal in January.
Earlier this month the US firm said the request had forced it to delay a shareholder vote on the proposed takeover until May 12, explaining that it will need ht additional time to provide the FTC with documents it wants.
When Steris’ proposed takeover of Synergy Health was announced last year US ratings agency Moody’s said Sterigenics “will face increasing competitive pressure over the medium to long term” as a result.