Two-dimensional (2D) layered materials have been an exciting frontier for exploring emerging physics at reduced dimensionality, with a variety of exotic properties demonstrated at 2D limit. Here, we report the first experimental discovery of in-plane antiferroelectricity in a 2D material β^{'}-In_{2}Se_{3}, using optical and electron microscopy consolidated by first-principles calculations. Different from conventional 3D antiferroelectricity, antiferroelectricity in β^{'}-In_{2}Se_{3} is confined within the 2D layer and generates the unusual nanostripe ordering: the individual nanostripes exhibit local ferroelectric polarization, whereas the neighboring nanostripes are antipolar with zero net polarization. Such a unique superstructure is underpinned by the intriguing competition between 2D ferroelectric and antiferroelectric ordering in β^{'}-In_{2}Se_{3}, which can be preserved down to single-layer thickness as predicted by calculation. Besides demonstrating 2D antiferroelectricity, our finding further resolves the true nature of the β^{'}-In_{2}Se_{3} superstructure that has been under debate for over four decades.