Can a Illinois landlord raise rent during a fixed-term lease?

Generally no. A fixed-term lease in Illinois locks the rent amount for the lease term. A landlord cannot raise rent during a fixed-term lease without the tenant's written agreement, regardless of whether the market rent has changed. On a month-to-month tenancy or after the fixed term expires, the landlord may increase rent with the required advance notice. Illinois has no statewide advance notice specific to rent increases. 735 ILCS 5/9-207 sets 30 days as the month-to-month termination default, which landlords treat as the minimum for rent changes. No state rent cap; local rent control is preempted statewide.

Source: Lease-based; 735 ILCS 5/9-207 (30-day month-to-month termination default)