"This request is based on many considerations, the foremost being my family, and the other being the simple realization that it is time for someone else to lead," he wrote.
Geeslin, 41, said he has no plans once he leaves the Texas Department of Insurance.
Geeslin, appointed by Perry in June 2005, has overseen a period of stabilization in the home insurance market after some rocky years as a result of massive claims for mold and water damage in the state.
Texas still has the highest homeowner rates in the nation, but premium increases have been moderate in recent years. On auto insurance, the state has the 13th highest rates in the nation, according to figures compiled by the National Association of Insurance Commissioners.
Geeslin has been unable to resolve a longstanding impasse with State Farm over alleged overcharges dating back to 2003. The dispute is still tied up in the courts.
In health insurance, he recently approved rules that eliminated the blanket authority of health, life and disability insurers to decide what their policies cover and don't cover in Texas.
The governor praised Geeslin's work as commissioner, Perry spokeswoman Catherine Frazier said. She said Perry hasn't yet appointed a replacement.
Geeslin was an aide to Perry who specialized in insurance and regulatory matters before he served as deputy commissioner for policy at the Department of Insurance.