Tujuan penelitian ini adalah untuk mengetahui sebaran klorofil-a serta kualitas air di Sungai Krueng Raba Lhoknga, Aceh Besar. Penelitian ini dilakukan pada Bulan Mei 2016. Pengambilan sampel dilakukan dengan cara purpossive sampling pada 5 stasiun pengamatan yang mewakili daerah sungai, muara dan laut. Hasil penelitian menunjukkan bahwa, konsentrasi klorofil-a tertinggi diperoleh di stasiun 3 (muara sungai) sebesar 4,55 µg/l dan terendah di stasiun 4 (laut) sebesar 0,43 µg/l. Kelas Bacillariophyceae mendominasi di Sungai Krueng Raba, dengan kelimpahan tertinggi diperoleh sebesar 603,28 Ind/l. Kata kunci: Klorofil-a; Kualitas air; Sungai Krueng Raba The objective of this research was to figure out the distribution of Chlorophyll-a and the water quality of Krueng Raba River in Lhoknga, Aceh Besar. The research was conducted on May 2016. Sampling was carried out using purpossive sampling method at 5 stations included the river area, estuary and inshore. The result showed the highest chlorophyll-a concentration was obtained at station 3 (estuary) and the lowest was at station 4 (inshore) with numbers respectively 4.55 and 0.43 µg/l. Bacillariophycea dominating the river with the highest value of density was 603.28 Ind/l. Keyword: Chlorophyll-a; Water Quality; Krueng Raba River