To rotate a point \((x, y)\) counterclockwise by 270 degrees about the origin, you can use the rotation formula:
\[ (x', y') = (y, -x) \]
In this case, the point is \((5, 8)\). Plugging these values into the formula gives:
\[ (x', y') = (8, -5) \]
So, the new point after rotating \((5, 8)\) by 270 degrees counterclockwise about the origin is \( (8, -5) \).
Thus, the answer is \((8, -5)\).