Ward's contract at New York Stadium had been due to expire this summer before the Millers exercised a clause which extended his deal by a further 12 months.
However, Sky Sports News reports that the newly-relegated League One side have now agreed to sell the 25-year-old to Championship outfit Cardiff for £1.6m plus add-ons.
Official confirmation is expected from the Bluebirds on Wednesday morning, with Ward set to become Cardiff's fifth signing of the summer already.
Newly-promoted Huddersfield Town will be entitled to 50% of the fee due to a sell-on clause in the contract which took him from the Terriers to Rotherham in 2015.
Ward made 41 appearances for Rotherham last season, scoring 10 goals.