The 6ft 6in centre half is likely to be loaned out to the EFL lower divisions in the near-future to broaden his footballing education in 2022-23.
The Sheffield-born player came through the Millers academy and signed his first professional contract in August 2020.
He had loan spells away in non-league at Matlock Town and Guiseley to boost his development before joining League Two side Hartlepool United in the January transfer window, making seven appearances for Pools.
Earlier on last term, Hull made his Millers' first-team debut, scoring in the 6-0 rout of neighbours Doncaster Rovers in the Papa Johns Trophy and also netting in the 5-0 win over Manchester City U21s in the group stages.
Hull was out of contract in June, but Rotherham exercised a one-year extension clause on his deal. - which was scheduled to run until June 2023.
Further discussions have now led to him signing a new long-term contract.