Japan’s SoftBank Group Corp. is pumping $1 billion into OneWeb Ltd., which aims to use hundreds of satellites to provide internet access in rural areas and developing countries, in a fundraising round that values the startup at roughly $2.5 billion.

OneWeb’s current investors are also giving $200 million in the latest fundraising, the company announced Monday. The fresh funds ensure that OneWeb has the firepower to build and launch more than 640 satellites by the beginning of the next decade. The company’s goal is to offer...