The First Ohio General Assembly was the initial meeting of the Ohio state legislature, composed of the Ohio State Senate and the Ohio House of Representatives. It convened in Chillicothe, Ohio, on March 1, 1803, and adjourned April 16, 1803.^[1] This General Assembly coincided with the first year of Edward Tiffin's first term as Ohio Governor.

Under Ohio's first constitution, State Senators were elected to two year terms. For the first class, half were elected for one year and half for two years. Members of the House were elected for each term. The Constitution was written in November, 1802, and submitted to the U S Congress. Elections for the first session were held in January, 1803. The apportionment of legislative districts was based on the 1802 Ohio Constitution, in section 7 of the Schedule.^[1] Subsequent sessions would be elected each October, and meet the first Monday of December.

In joint session, state officers were elected: William Creighton Jr. as Ohio Secretary of State, William McFarland as Ohio State Treasurer and Thomas Gibson as Ohio State Auditor.^[6]

John Smith and Thomas Worthington were elected United States Senators, with no record of vote made.^[6]

In joint session, local judges and three Ohio Supreme Court judges, Return Jonathan Meigs Jr., Samuel Huntington, and William Sprigg, were elected.^[7]

Eight new counties were erected during this session:^[7]

• Butler detached from Hamilton
• Columbiana erected from parts of Jefferson and Washington
• Franklin detached from Ross
• Gallia from Ross
• Greene from parts of Hamilton and Ross
• Montgomery from Hamilton
• Scioto from Adams
• Warren from Hamilton

Existing territorial laws were recognized as in force, if not specifically invalidated.

• List of Ohio state legislatures