The 47-year-old died on June 25, 2010 at their home in Bassingbourn, Cambridgeshire, the force said.
A re-investigation into her death began after Stewart was convicted in 2017 of the murder of his fiance Helen Bailey at their home in Royston, Hertfordshire in 2016.
Stewart is due back before the court for a plea hearing on September 21.
Cambridgeshire Police also said in a statement: “Ian Stewart, 59, appeared at St Albans Crown Court today (6 July) charged with the murder of his wife Diane.
“The 47-year-old died on 25 June 2010 at their home in Bassingbourn.
“A re-investigation into her death began after Ian Stewart was convicted (2017) of the murder of his Fiancé Helen Bailey at their home in Royston in 2016.
“A plea hearing has been set for 21 September.”