| Competition: | Welsh Football League Division Three |
| Season: | 2009–10 |
| Winners: | Aberbargoed Buds |
| Promoted: | Aberbargoed Buds, Abertillery Bluebirds, Cwmaman Institute |
| Relegated: | Seven Sisters, Troedyrhiw, Llantwit Fardre, Pentwyn Dynamo, Garw |
| Biggest Home Win: | Cwmaman Institute 6–0 Garw 15 May 2010 |
| Biggest Away Win: | Garw 1–6 Goytre 26 September 2009 Garw 1–6 Seven Sisters 10 October 2009 Pentwyn Dynamo 1–6 Pontyclun 10 October 2009 South Gower 1–6 Abertillery Bluebirds 13 February 2010 |
| Highest Scoring: | Cwmamman United 5–6 Briton Ferry Llansawel 19 August 2009 Goytre 7–4 Garw 23 January 2010 South Gower 7–4 Garw 13 April 2010 |
| Total Goals: | 1105 |
| Average Goals: | 3.61 |
| Prevseason: | 2008–09 |
| Nextseason: | 2010–11 |
The 2009–10 Welsh Football League Division Three began on 15 August 2009 and ended on 22 May 2010. Aberbargoed Buds won the league by one point.
Abertillery Bluebirds were promoted from the Gwent County League and Corus Steel were promoted from the South Wales Amateur League. Briton Ferry Athletic and Llansawel merged to form Briton Ferry Lansawel.[1] South Gower applied to join the league and were accepted replacing Llansawel.[2]
Ystradgynlais were relegated to the Neath & District League and Merthyr Saints were relegated to the South Wales Amateur League.
AFC Llwydcoed, AFC Porth and Porthcawl Town were promoted to the Welsh Football League Division Two.
Pentwyn Dynamos, Garw and Pontypridd Town were relegated from the Welsh Football League Division Two.