The mirrorless camera with the highest resolution is the Sony α7R II, with a resolution of 7952 x 5304 (42.4 megapixel).
Sony α7R II
The α7R II combines the world’s first back-illuminated 42.4 MP 35 mm full-frame CMOS image sensor, Fast Hybrid AF, 5-axis image stabilization and advanced 4K movie recording, with sensitivity and speed to keep even fast-moving subjects looking sharp.
This camera is also known as the A7R II,ILCE7RM2, ILCE7RM2/B
[as of March 2016]