I would definitely try to win.
Should I let my girlfriend beat me in a chess tournament?

So I was entering a chess tournament tommorow, and my girlfriend felt bored and wanted to join. Only problem is that she is a complete begginer and doesn't know any tactics.
The worst part is that I'm actually against her tommorow. Should I purposely let her beat me, or should I play an honest game? If so, how can I lose without it looking intentional?

Go through some basic opening prep with her tonight, and maybe she will survive into the middle game tomorrow.

Play honest game. She gains nothing if you let her intentionally win, nor will you.
+1
I would be more insulted if I found out that someone let me win, than I would be if they beat me fairly. Letting someone win is no different from lying.

No. You slaughter her, and then later you go over the game with her and help her learn from it. Never lose on purpose in chess.
Go ahead and beat her. She knows she is a beginner. A self-respecting person would feel condescended to by someone who deliberately loses, and she will know. If losing to her would be a successful way to curry favor with her, than get yourself a girlfriend with better character.

So I was entering a chess tournament tommorow, and my girlfriend felt bored and wanted to join. Only problem is that she is a complete begginer and doesn't know any tactics.
The worst part is that I'm actually against her tommorow. Should I purposely let her beat me, or should I play an honest game? If so, how can I lose without it looking intentional?
Play an honest game. If you let her win and she knows you let her win, she may lose all respect for you. I mean, after the game, the sandwich isn't going to make itself.

You would get more chances later in the tourney- so play normally tomorrow. You can share your prize at the end with her. :)

Aside from the issues of competitive integrity that others have mentioned, you'd just be setting her up for an embarrassing loss to a stranger. That can't end well for anyone.

Letting her win won't improve your skill or her's. You would both benefit from not trying to throw the game.

Do what the Kosintseva sisters would do in that situation. :-)
Is it a knockout tournament?
So I was entering a chess tournament tommorow, and my girlfriend felt bored and wanted to join. Only problem is that she is a complete begginer and doesn't know any tactics.
The worst part is that I'm actually against her tommorow. Should I purposely let her beat me, or should I play an honest game? If so, how can I lose without it looking intentional?