| Competition: | Cymru Alliance |
| Season: | 2009–10 |
| Winners: | Llangefni Town |
| Relegated: | Holyhead Hotspur, Bethesda Athletic, Denbigh Town, Llangollen Town, Berriew, Mold Alexandra, Lex XI, Llanfairpwll, Caernarfon Town & Gresford Athletic |
| League Topscorer: | 28 – Shaun Beck (Flint Town United)[1] |
| Biggest Home Win: | Llangefni Town 8–0 Llangollen Town 10 April 2010 |
| Biggest Away Win: | Penrhyncoch 0–8 Flint Town United 24 October 2009 |
| Highest Scoring: | Llangollen Town 6–4 Mold Alexandra 27 February 2010 |
| Total Goals: | 963 |
| Average Goals: | 3.54 |
| Prevseason: | 2008–09 |
| Nextseason: | 2010–11 |
The 2009–10 Cymru Alliance season began on Friday 14 August 2009 and ended on Saturday 8 May 2010. The league was won by Llangefni Town.
Bala Town as 2008–09 Champions were promoted to the Welsh Premier League with Caernarfon Town replacing them.
Berriew were promoted from Mid Wales League, Bethesda Athletic were promoted from the Welsh Alliance League and Llangollen Town were promoted from the Welsh National League.
Llandyrnog United were relegated to the Welsh Alliance League.
Mynydd Isa[2] and Glantraeth[3] both resigned from the league.