The Isthmian League First Division (also known for sponsorship reasons as the Ryman League First Division) is part of the Isthmian League, an English association football league. The first division ranks below the league's Premier Division and above a variety of local leagues in southeastern England including the Isthmian's own Second Division.

As part of the 2004 restructuring of the National League System, the two First Divisions of the Isthmian League that had existed since 2002 are being recombined to become one of four "Step 4" leagues of the NLS, making them part of the eighth overall tier of the English football league system. Promotion from the Isthmian First will be to either the Isthmian Premier or the Dr Martens Southern League Premier Division depending on a team's geographic location. Top-placed teams meeting the necessary ground and financial standards will be promoted, along with teams emerging from playoffs.

The May 9, 2004 restructuring meeting of The Football Association assigned the following teams as the members of the Isthmian League First Division for the 2004/2005 season:

• AFC Wimbledon
• Ashford Town F.C. (Kent)
• Banstead Athletic F.C. [1]
• Bashley F.C. [2]
• Bromley F.C. [3]
• Burgess Hill Town F.C. [4]
• Corinthian Casuals F.C. [5]
• Cray Wanderers F.C. [6]
• Croydon Athletic F.C. [7]
• Croydon F.C.
• Dorking F.C. [8]
• Dulwich Hamlet F.C. [9]
• Hastings United F.C. [10]
• Horsham F.C. [11]
• Leatherhead F.C. [12]
• Metropolitan Police F.C.
• Molesey F.C.
• Newport IOW F.C. [13] [14]
• Sittingbourne F.C. [15]
• Tooting & Mitcham United F.C. [16]
• Walton & Hersham F.C. [17]
• Whyteleafe F.C. [18]

In June 2004 it was announced that Sittingbourne would be returning to the Southern League and Fleet Town F.C. [19](relegated after 2003/2004) would replace them in the Isthmian League First Division.