To get button no border effect in tkinter tk I used to set borderwidth=0
What you want can be achieved by setting the button relief to flat
or borderwidth to 0 using a ttk style. However, some ttk themes don't take into account these style settings, and one of them is the default theme in Windows. Setting the theme to 'clam' or 'alt' should solve your problem.
import tkinter as tk
from tkinter import ttk
root = tk.Tk()
b1 = tk.Button(root, text='tk.Button', borderwidth=0)
b1.pack()
s = ttk.Style(root)
s.theme_use('clam')
s.configure('flat.TButton', borderwidth=0)
# s.configure('flat.TButton', relief='flat') gives the same result
b2 = ttk.Button(root, style='flat.TButton', text='ttk.Button')
b2.pack()
root.mainloop()
You can't remove the border on windows or osx. The whole point of using ttk buttons on those platforms is to get native widgets. If you want a button without a border, use the standard tk button.