Sicklasjön ( Swedish: "Lake of Sickla") is a lake in eastern central Stockholm, Sweden. It is bordering the municipalities of Stockholm and Nacka and is named for the vicinity to the urban district Sickla.
Sicklasjön, historically known as Långsjön ("Long Lake"), is connected to Järlasjön east of it through a narrow strait and to Hammarby Sjö west of it through the canal Sickla Kanal and the sluice Sickla Sluss. The lake, forming the northern border of the Nacka Open-air Area (Nacka friluftsområde, colloquially known as Nackareservatet, "Nacka [Nature] Reserve"), is considered as of significant recreational and natural value. Water flow is periodically important but inconsiderable during summers when the lake remains relatively stratified.