Many managers across all four divisions complain every year about the summer window shutting after the season has got under way.
But Dier has joined in the criticism and claims it is a problem for the players too as they try to begin the campaign well.
Spurs, who sold midfielder Ryan Mason to Hull for just over £10m yesterday, have won just one of their three games this term and Dier feels uncertainty over squad members doesn't help.
He said: "My opinion is the transfer window shouldn't be open now, it should be over once the Premier League season begins.
"I just think it's unfair on managers and players that there are still people in the dressing room that don't really know what their futures are.
"Once the season starts, I want to be at a club where everyone is pushing in the same direction and knows where they stand.
"I don't think it helps players or managers.
"Obviously it's good entertainment for everyone. I enjoy watching it, but it shouldn't be open when the season starts."
