Am I required to sign a severance agreement in California?

You can't be required to sign a severance agreement, in California or any other state. However, your employer may be able to condition your severance pay on your signing. In other words, if you want to get severance pay, you may have to sign the agreement. 

Many severance agreements include a release or waiver: an agreement not to sue your employer for legal disputes arising out of your employment. This is what the employer wants in exchange for paying you severance. 

A release is valid only if it is voluntary. If you are forced or required to sign, the agreement won't be enforceable. However, employers are generally free to offer severance only to those employees who sign a release. Even if you really need that severance pay, courts don't view this type of requirement as making the agreement involuntary. 

The exception to this rule is for employees who are already owed severance. For example, if you have an employment contract stating that you will receive six months' severance if you are fired before your contract expires, you are entitled to that money. The employer can't require you to sign a release to get it, because you would be giving up your right to sue without receiving anything in exchange. If an employer in this situation wants a release, it would have to offer you more than you are already owed. For example, the employee with the contract is already entitled to six months of severance, so the employer might offer an additional three months of pay if the employee signs a release.