Official government estimates show a gradual decline in union density in Japan over several decades akin to that in other countries with decentralized bargaining structures. However, new evidence from various social surveys indicates that union density has been rising in Japan. Using one of these social surveys – the Survey on the Work and Life of Workers (SWLW) – we show union density has risen by 7.3 percentage points to 29.1% in the Japanese private sector between 2011/13 and 2020/24. We decompose the growth in union density since 2011/13 to establish how much of it is attributable to changes in workforce composition. Conditioning on union presence at the workplace, compositional change accounts for 47% of the increase in union density. The remaining 53% is due to within-group change with unions increasing membership across all types of worker including some with traditionally low rates of unionization. However, establishing a union at the workplace remains key since virtually all the growth in union membership (97%) is in unionized workplaces.