And in a seeming attempt to drill in her point, Bynes also attached a photo of Cyrus at the recent Billboard Music Awards.
Cyrus, for her part, has so far remained mum about Bynes' social-media attack.
This is yet the latest Twitter tirade unleashed by the embattled Bynes, who was arrested last month after allegedly throwing a bong out of her apartment window.
And Cyrus wasn't Bynes' only online target earlier today: After lobbing a slew of "ugly" comments at both Drake and her father this week, the 27-year-old once again took a shot at Drake.
"Is that an ugly face or what?" she tweeted, attaching a photo of Drake.
Bynes also remained indignant about her posts, later adding: "This is my twitter. I say whatever I want. Thanks!"
But in a quick change of pace, the star perked up when it came to plugging a few projects she seemed to have in the works.
"I'm so looking forward to recording my album and going on tour! I can't wait to see you all at my concerts!" she wrote.
Guess we won't be seeing Cyrus or Drake there.