Forward-looking　sonar　(FLS)　and　camera　are　widely　used　sensors　in　multimodal　systems　for　underwater　environments.　The　extrinsic　calibration　is　a　precondition　for　this　kind　of　multimodal　systems　to　fuse　optical　and　acoustic　information.　Due　to　the　very　different　imaging　formation　mechanisms　and　models　of　3-D-to-2-D　projection　for　imaging　sonar　and　optical　camera,　the　multimodal　correspondence　and　the　sonar-pose　recovery　from　a　2-D　sonar　image　are　two　main　issues　in　extrinsic　calibration　work.　In　this　article,　we　propose　an　extrinsic　calibration　method　for　the　opti-acoustic　imaging　system　combining　a　camera　and　an　FLS　in　underwater　environment.　The　proposed　method　includes　an　estimation　process　yielding　closed-form　expressions　to　determine　the　sonar　pose　with　respect　to　the　calibration　board,　which　enables　the　estimation　of　extrinsic　parameters　to　be　directly　computed　from　sensor　measurements　and　the　knowledge　of　calibration-board　structure.　A　calibration　board　with　two　sets　of　feature　arrays　is　used　to　generate　two　2-D-to-3-D　correspondences,　which　circumvents　the　2-D-to-2-D　multimodal　correspondence　problem.　An　iterative　optimization　process　is　provided　to　further　improve　the　performance,　while　the　extra　computation　time　is　negligible.　Both　simulated　and　actual　experiments　show　that　the　method　outperforms　the　existing　iterative　method　based　on　bundle　adjustment.