(for Godot 4) If using a Sprite3D node, under the geometry category of the inspector, you can drag and drop your sprite onto the Material Override property. After that, enable Transparency (if needed) and then under Sampling, change the filter to Nearest
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(for Godot 3) Next to the scene tab is "Import". Once in there,
find the problematic image file that's blurry,
deselect "Filter", and reimport.
For blurry sprites used in earlier versions of Godot I suggest
this video: https://youtu.be/FgtekgMca2E?t=254