Hard Mode is an optional difficulty-enhancing game mode, triggered by naming the fallen human "Frisk". Similarly to the Undertale Demo, Hard Mode lasts only until the end of the Ruins. It consists of slight dialogue changes, and much more difficult enemy encounters. At the end of the boss battle with Toriel, the Annoying Dog arrives, announcing the end of Hard Mode, much to the displeasure of Toriel. Some monsters are exclusive to Hard Mode while others can be found later in the game in the CORE outside of Hard Mode.
Attempting to escape the Ruins during Hard Mode by editing the protagonist's location in their save file, or by direct intervention (cheat engine, debug mode, etc.) will cause the game to continue normally.
- Different, more difficult enemies replace the enemies that appear in a normal run. For example, Froggits are replaced with Final Froggits.
- The battle theme is different, using "Stronger Monsters" instead of "Enemy Approaching"
- The labels for the switches have worn away.
- The first Froggit in the room after Toriel leaves tells the protagonist that running away does not work on the first turn.
- Only three pieces of Monster Candy can be taken instead of 4; the flavor text appears: "In this hellish world, you can only take three pieces of candy..."
- When Toriel leaves Home, she does not bring groceries.
- The end of Toriel's boss fight is interrupted by the Annoying Dog, and the rest of the game is inaccessible.
- If Toriel is struck down during her fight, she stops dying once Annoying Dog interrupts.
- Even if Toriel is spared, the Annoying Dog still asks Toriel if she's supposed to be dying. Not so smart anymore, are you Toby?
- When the game is reset, the name of the fallen human can be changed.
- This was probably done to prevent the player from having their game stuck on Hard Mode forever, as it is triggered by choosing a certain name.
- Annoying Dog: And that's the end of Hard Mode!
- Toriel: Eh?? You are ending it NOW? And on such a dramatic moment...?
- Annoying Dog: That's the difficult part. Not the bullets. But, accepting that it's all over...
- Toriel: But there WILL be more, will there not?
- Annoying Dog: Maybe. Knowing the answer is... ... HARD. ...
- [Toriel stands back up (If defeated).]
- Annoying Dog: Hey! Aren't you supposed to be dying or something?
- Toriel: Well. What is the point of that now?
- Annoying Dog: What will you do instead...?
- Toriel: Hmmm. Perhaps I will bake another pie. That last one ended up a little burnt.
- Annoying Dog: I thought it was good. Theoretically. It's not like I ate it all while you were fighting.
- [Toriel walks away angrily.]
- Annoying Dog: Hey! Hey! Can I have some pie!
- Toriel: You are just going to eat it all...
- Annoying Dog: I can helllp!!!
- Toriel: Snoring on the floor is NOT help.
- Annoying Dog: I'm not snoring, I'm cheering you on in my sleep!! ... Oh, you're still here? Don't you have anything better to do?
- [Annoying Dog exits the screen.]
- [Rock sounds and the following text appears on the Undertale logo:]
- HARD MODE
- Eh. don't count on it.
- Flowey: Hey. What's the hold up!? Shouldn't she be dead by now!? I've been waiting in that room for... ... ... Hard mode...? Gee, you better take a picture. People are gonna think you´re REALLY cool. ... NOT! Golly, talk about a tryhard. PA. THE. TIC. Uh, so, are you gonna keep going, or...?
- Toriel (talking from outside of the screen): It's over.
- Flowey: Hahaha! I KNEW that! Why does everyone have to be so condescending!?... So what's YOUR excuse? Sitting around here...? Don't you have anything better to-
- Annoying Dog: [Interrupting] I already said that.
- [Flowey smiles and disappears into the ground. The text fades away.]
- Completing Hard Mode on the PS4 and Nintendo Switch versions of the game grants the player a humorous border labeled "Beauty", featuring muscular humanoid versions of the Annoying Dog, Flowey and Toriel.