Can an employer legally reduce my pay after hiring without my consent?

Location: California, USAI was hired a few months ago for a full-time position and was given a written offer letter stating my hourly pay. I accepted the offer and have been working under those terms since then.

Last week, my employer told me that due to “budget issues,” my hourly pay will be reduced starting next pay period. I was not asked to agree to this change, just informed that it would happen. There is no new contract, and I have not signed anything agreeing to the lower rate.

I’m still performing the same duties, same hours, and nothing about my role has changed.

I’m trying to understand:

• Is it legal for an employer to reduce pay like this going forward?
• Does the written offer letter have any legal weight?
• Do I have any options if I don’t agree to the new rate?

I’m located in the U.S. (at-will employment state).

Any general guidance would be appreciated.