This study evaluated methane (CH₄) emissions from livestock and rice cultivation in Cagayan Valley, Philippines, using the IPCC Inventory Software and the 2006 IPCC Tier 1 methodology to address the need for localized greenhouse gas (GHG) inventories. Using data from the Philippine Statistics Authority, CH₄ emissions for 2020 and 2021 were quantified, analyzed, and compared with national and international inventories. Results indicated a slight decline in emissions from 44.32 Gg in 2020 to 43.53 Gg in 2021 due to enteric fermentation, and a more notable decline, from 10.04 Gg in 2020 to 5.69 Gg in 2021 in manure management emissions, likely due to improved livestock management. However, rice cultivation emissions increased from 77.81 Gg in 2020 to 80.61 Gg in 2021, suggesting the need for targeted mitigation strategies. Cattle and buffalo were the primary livestock sources of CH₄, while swine contributed the most to manure management emissions. The findings emphasize the importance of accurate, region-specific GHG inventories for effective climate action. The study serves as a model for other agriculture-intensive regions seeking to quantify and reduce GHG emissions.