The court decided the £84m deal could go ahead following a dispute over a shareholder vote.
Some staff and customers had opposed the move amid fears for jobs and bills.
But the court adjourned until 10 February, pending any application to appeal - so the scheme will not become effective until then.
Dee Valley is due to make a statement.
Rhys McKenzie, a former employee and shareholder, expressed "severe disappointment" at the judge's ruling.
He said he was worried about his former co-workers and their families, but also for "the small shareholder around the country".
Before shareholders of the Wrexham-based company met to vote on the proposed takeover, about 450 customers and staff had shares transferred to them.
The aim was to try and keep the company out of the hands of the Coventry-based provider.
The court was asked to decide whether those votes were valid, and ruled the takeover could still go ahead.
Plaid Cymru AM Llyr Gruffydd said the takeover was "not good news".
"Jobs will be lost and the local supply chain will suffer.
"Small shareholders have been disregarded by the judgement while big corporate shareholders will be laughing all the way to the bank due to today's ruling."
A spokesman for Dee Valley Water previously described the case as "unprecedented in the UK" and as "a David versus Goliath battle between local staff and customers versus a FTSE 100 company".
Dee Valley Water employs about 180 people and has 230,000 customers across the Wrexham and Chester areas.
Customers pay an average of £145 a year for their water compared with £172 for Severn Trent.
Severn Trent's bid to take over Dee Valley Water has been given the go-ahead after a "David versus Goliath" dispute in the High Court.