In a Tuesday court filing, attorneys for Holmes said they plan to formally ask for a change of plea at a May 13 hearing.
A not-guilty plea had previously been entered on Holmes' behalf by a judge. That judge must approve the insanity plea before the plea can be changed.
In court documents and hearings, attorneys had said that Holmes is mentally ill. He was seeing a psychiatrist before the mass shooting last summer.
Holmes is accused of entering a darkened movie theater, throwing explosives and shooting at people, killing 12 and injuring 70 in Aurora, Colorado, on July 20, 2012.
If convicted of more than 160 counts of murder and attempted murder, Holmes could be executed.
The Associated Press contributed to this report.