Naismith led Hearts to third place in the Scottish top flight last season but they are rock bottom in the current campaign, with just one point from six matches after a 2-1 defeat by St Mirren.
Chief executive Andrew McKinlay said the club had taken the "difficult decision" to part ways with Naismith, who just last month agreed a contract extension until 2026.
"Things can change quickly, particularly in football, and it has been clear for all to see that things are not working this season," McKinlay said in a statement.
"Results and performances have fallen below the standards expected at this football club and, unfortunately, there is little evidence of any potential upturn in fortunes."
Speaking after the St Mirren defeat on Saturday, Naismith appeared to realise that the writing was on the wall.
"I'm realistic to understand that the more games we pass, the more pressure builds," he said.
"It will be what it'll be. The one thing I've felt my whole time at the club as a player, as a coach and as a manager, is that the board are realistic, they're sensible, they understand it. They make good decisions."
