Spatial　relationships　between　forest　and　PM2.5　concentrations　are　of　great　policy　implications　in　regional　afforestation　layout　and　air　pollution　control.　This　paper　investigates　the　transboundary　externality　of　a　city＇s　forest　on　the　concentrations　of　PM2.5　in　different　city　segments.　Employing　a　mixed-regressive　spatial　panel　model　with　data　for　255　Chinese　cities　over　2000　to　2015,　we　find　that　the　concentrations　of　PM2.5　tend　to　be　substantially　lower　in　cities　with　larger　forest　area　and　the　depositing　effect　of　forest　spills　over　significantly　to　neighboring　cities.　A　one　percentage　increase　in　forest　area　reduces　the　average　annual　concentrations　of　PM2.5　by　2.53%,　of　which　76%　is　contributed　to　the　spillover　effect.　Moreover,　the　average　marginal　effect　of　forest　on　PM2.5　concentrations　exhibits　an　inverted-U　relationship　with　wind　speed　and　the　depositing　effect　minimizes　(in　magnitude)　as　the　average　annual　speed　of　wind　approaches　to　23　kilometers　per　hour.　These　findings　suggest　that　severe　hazing　cities　with　mild　wind　speed　are　priority　afforestation　areas　for　transboundary　air　pollution　control.　©　2020　Elsevier　B.V.