The move comes in the month Scarlets announced a £2.1m loss for the latest financial year.
Executive chairman Simon Muderack said “Signing the PRA25 is another important milestone as we build the next chapter in Scarlets’ proud 150-year history.
“With certainty for the next two years, we can accelerate the significant progress already underway on and off the field and move forward with confidence.”
However the deal only guarantees the future of all four regions until June 2028.
The governing body still intends to reduce the number of men’s professional sides from four to three – with one club in the east of Wales, one in Cardiff and one in the west – though announced a delay in revealing how exactly it plans to do so.
The decision has been heavily criticised by players, fans and politicians and has sparked turmoil, protests and even legal action.
WRU chief executive Abi Tierney added: “We have now entered a period of increased stability across the performance game in Wales and begun to increase investment in our support structures and pathways.
“A lot of hard work is going into our off-the-field activity to create an enhanced professional environment for Welsh players at every touch point that matches their on-field commitment.
“Our strategy for the long-term future is clear and there will be further updates on our progress in this area soon.”
