STC signed a non-binding agreement in January to buy the majority stake for $2.4 billion, but extended the process in April by 90 days, citing logistical challenges caused by the coronavirus pandemic.
In a statement, STC said it was extending this period again until September for the same reasons.
Vodafone Egypt is the country's biggest mobile operator with 44 million subscribers and a 40% market share.
STC is majority owned by Saudi Arabia's state fund the Public Investment Fund (PIF).
(Reporting by Alexander Cornwell; Editing by Raissa Kasolowsky)