SoftBank will plough $25bn into the Vision Fund, while Apple has committed to putting in $1bn.
"We believe their new fund will speed the development of technologies which may be strategically important to Apple," said Apple spokesman Josh Rosenstock.
The new round of investment commitments will allow the group to hit its $100bn goal weeks ahead of schedule.
City A.M. understands the core operations of the fund have already been set up and SoftBank has begun to evaluate what projects it will work on.
SoftBank has reportedly been in talks with Saudi Arabia's Public Investment Fund for a commitment that could reach $45bn. Abu Dhabi's sovereign wealth fund has also been in talks with the Japanese firm, the Financial Times reported.
The fund, almost unprecedented in its size, is expected to help plug a late stage funding gap for startups across Europe.
SoftBank's chairman and founder Masayoshi Son has previously said he wants to become the Warren Buffett of tech.
The telecoms group snapped up UK chip maker Arm Holdings in a more than £24bn pound deal last year.