The package, among the biggest in German corporate history, still needs approval from the European Commission and the backing of Uniper shareholders.
It carries certain conditions, including that Uniper withdraw a lawsuit against the Netherlands over its coal phase-out as well as a commitment by the Duesseldorf-based group to suspend dividend payments for the duration of its stabilisation period.
The support of Germany's federal government contributed to S&P's decision to affirm the rating, it said in a statement.
The outlook negative rating indicates a potential downgrade is less imminent or likely than was the case under Uniper's previous rating of "CreditWatch negative."
"This outlook reflects prevailing uncertainty surrounding gas flows and the fact that details of the stabilisation package are still to be approved," S&P said.
($1 = 0.9792 euro)
(Reporting by Victoria Waldersee in Berlin; Editing by Matthew Lewis)