Seeds, essentially random strings of characters, are used as the basis for all decision making in the game. By inputting a seed manually, one can guarantee that most randomly-generated decisions will occur the same every time. For instance, two players using the same seed will find the same items in the same locations.
Seeds are the same on every platform as long as the game version is also the same. For instance, if you find a Rebirth seed on PC, you can use it on the 3DS version and it will be the same because the 3DS version only has Rebirth.
Except for some special seeds, inputting a seed will cause the 'no trophies' mark to appear. This prevents any achievements from being obtained on that particular run.
All alphanumeric characters are accepted except for 5, I, O, and U. On the PC version pressing the I, O, and U keys during seed entry replace them with 1, 0, and V respectively.
Once a special seed has been entered, it will be permanently unlocked without needing to enter the code again. Pressing Q / Left Bumper during character selection will bring up the Easter Egg Section, in which any previously unlocked game mode can be selected. Selecting multiple modes at once is also possible.
On the special seed selection menu, it is possible to remove a discovered seed by focusing it and pressing Q+E+Tab on keyboard or LB+RB+X on controller.
Pickups quickly fade. This includes pedestal items, shop purchases, devil deals, and the Cathedral/Sheol boss chest.
33
FREE 2PAY
F2P version
Isaac starts with 69 coins, but all items and consumables must be bought.
25
PAC1 F1SM
Pacifism
Enemies do not take damage. All rooms are considered cleared upon entering them; doors stay open, room rewards spawn, Stonies instantly die. When in a boss room, the boss's health bar is used as a timer and slowly ticks down. At the end of the countdown, the trapdoor to the next floor appears.
32
C0ME BACK
Enemies respawn
Enemies, including bosses, respawn once Isaac leaves the room. Completing a room multiple times does not yield additional pickups.
71
N0RE TVRN
Point of no return
Boss rooms lock Isaac in, similar to the Mom fight.
24
BRWN SNKE
Poopy trail
Isaac leaves a trail of poop behind him as he walks.
Spawns a G FUEL! item in the starting room of each floor. Each copy picked up has different effects and adds various excessive "gamer style" visual and sound effects to the gameplay. Chapter 2 will always be Flooded Caves with neon-blue G Fuel pits. ??? will be replaced by a can of G Fuel.
Isaac takes a full heart of damage from all sources.
48
BRTL B0NS
Twelve heart damage
Isaac takes twelve hearts of damage from all sources.
26
D0NT ST0P
Don't stop moving
Isaac takes damage if he stops moving.
27
THEG H0ST
Damage on a timer
Every 45 seconds, a ticking will begin, and after 5 seconds, Isaac will take 1/2 heart of damage. Damage increases to 1 heart in Chapter 4 and beyond. Having invincibility during the time when damage should be inflicted blocks the damage.
28
30M1 N1TS
30 minute limit
Gives a time limit of 30 minutes to beat the game. When the timer hits 29:45, a timer starts ticking, and after 30:00, Isaac takes damage every second.
Control Modifiers
52
1CES KATE
Ice physics
Makes the ground slippery.
74
BRAV ERY
Bravery
Isaac automatically fires tears in the same direction that he moves. Removes the ability to fire tears manually.
75
C0WR D1CE
Cowardice
Isaac automatically fires tears in the opposite direction that he moves. Removes the ability to fire tears manually.
12
DRAW KCAB
Reversed controls
All movement and firing keys are reversed.
76
AX1S ALGN
Axis aligned controls
Isaac cannot move diagonally.
Camouflage
3
CAM0 K1DD
Camouflage player
Isaac blends into the ground.
4
CAM0 F0ES
Camouflage enemies
All enemies blend into the ground.
5
CAM0 DR0P
Camouflage stuff
Pickups and items blend into the ground.
6
WH0A WHAT
Camouflage EVERYTHING!
Isaac, enemies, pickups, and items all blend into the ground.
14
1MN0 B0DY
Invisible player
Isaac becomes completely invisible.
15
BL1N DEYE
Invisible enemies
All enemies become completely invisible.
Player Appearance
46
THEB LANK
The blank
Isaac has no face.
53
KAPP A
Grayscale
Isaac turns gray.
54
H0H0 H0H0
Christmas
Isaac wears a Christmas hat.
35
T0PH EAVY
Big head mode
Isaac's head is large.
36
T1NY D0ME
Tiny head mode
Isaac's head is small.
37
PTCH BLCK
Black body
Isaac and his tears turn completely black.
Visual Effects
38
TEAR GL0W
Glowing tears
Isaac's tears glow.
23
BL00 00DY
Extra gore
Makes a lot more blood appear after enemies die.
HUD Modifiers
8
B00B T00B
Cathode rays!
Applies a filter that makes the game look like it's being played through a cathode ray tube.
9
DYSL EX1A
Dyslexia
All in-game text is rearranged.
10
KEEP TRAK
No HUD
Removes the HUD entirely. This affects the map, health, consumables, held items, pickup text, floor names, and more.
While the BRTL B0NS seed is enabled, getting hit will not result in death if Isaac possesses Red Hearts as well as Soul Hearts or Black Hearts. Instead, the Soul hearts and/or Black hearts will be completely depleted, leaving Isaac with only his Red hearts. You can also prevent death with Bone Hearts, as they will get depleted without destroying the bone heart and protecting the rest of the hearts, and can be refilled with Red Hearts. The soul hearts in front of them will get destroyed though.
If the ICES KATE seed is enabled, purple champion monsters are be much more dangerous, as escaping their gravitational pull becomes almost impossible.
When playing with the KAPP A seed enabled, certain items will override the grayscale, such as Kidney Stone.
When the seed PACI FISM is enabled, effects that kill enemies on impact such as Euthanasia and Little Horn still remove enemies, but they will respawn when the room is re-entered. No effect occurs after killing all in a room.
Some seeds can not be enabled alongside other certain seeds. For example, the PTCH BLCK seed cannot be enabled alongside THEB LANK.
It is possible to access other chapters while playing the BASE MENT seed. Stepping into the beam of light after using Genesis can take Isaac to chapter II, avoiding the standard loop after the first two basement floors. Additionally, Sacrifice Rooms after 12 uses can be used at any point to teleport to the Dark Room, making it possible to see a proper ending and finish the run.
Some special seeds were shared with the community by the developers.
If an item that would appear on a seed is not unlocked yet, it will be replaced by another item that is unlocked.
The "DYSL EXIA" seed is a reference to dyslexia, a disorder that causes difficulties in spelling, reading, and writing.
The seeds "ANDA NTE", "LARG HET0", "ALLE GR0", and "PRES T0" refer to andante, larghetto, allegro, and presto, respectively. These are names of musical tempos, which are the speeds of given pieces of music.
The "KAPP A" seed is a reference to a commonly used Twitch emote "KAPPA".
The "ALM1 GHTY" seed's description is, "Dog Mode". This is referencing the term "God mode", which is used to describe someone or something in a game that has invincibility, usually attained through the use of cheats.
"Dog Mode" is also a reference to the 1994 first-person shooter game Rise of the Triad, which has a power-up of the same name.
The "FEAR M1NT" seed is a reference to spearmint, an herb popularly used for flavoring food.
Although the "T1NY D0ME" seed also makes Isaac's hitbox somewhat smaller and could be considered beneficial, it does not prevent achievements.
The "SVPE RH0T" seed is a reference to the 2016 first-person shooter game Superhot.
"M0DE SEVN" i.e. Mode 7 is a graphics mode on the Super Nintendo Entertainment System console that often results in pronounced pixelation.
"GFVE LLLL" is used as 11/4/2022 and 3/24/2023's daily runs.
While using Tainted Forgotten in the "SVPE RH0T" seed, time will only move when the skeleton moves.
Bugs
Bug!
Entering the Blue Womb while using the "N0RE TVRN" seed is not recommended as the door leading to the Boss Room will not spawn, thereby trapping the player and ending the run.
Bug!
Returning to the menu from a run that uses a seed that alters audio speed will cause the main menu audio to remain altered. This resets upon entering a new run, or exiting the game.
Bug!
The PAC1F1SM seed does not allow you to finish a The Beast run. Dogma never transforms in his second phase. If you manage to go to the Beast fight (with the debug console), the Chest spawns before Ultra Famine is killed.
Once a special seed has been entered, it will be permanently unlocked without needing to enter the code again. Pressing Q / Left Bumper during character selection will bring up the Easter Egg Section, in which any previously unlocked game mode can be selected. Selecting multiple modes at once is also possible.
Cathedral/
Sheol boss chest.
Stonies instantly die. When in a boss room, the boss's health bar is used as a timer and slowly ticks down. At the end of the countdown, the trapdoor to the next floor appears. 
Mom fight.
Flooded Caves with neon-blue G Fuel pits. 
??? will be replaced by a can of G Fuel. 
Red Hearts as well as 
Soul Hearts or 
Black Hearts. Instead, the Soul hearts and/or Black hearts will be completely depleted, leaving Isaac with only his Red hearts. You can also prevent death with 
Bone Hearts, as they will get depleted without destroying the bone heart and protecting the rest of the hearts, and can be refilled with Red Hearts. The soul hearts in front of them will get destroyed though.
Kidney Stone.
Euthanasia and 
Little Horn still remove enemies, but they will respawn when the room is re-entered. No effect occurs after killing all in a room.
Genesis can take Isaac to chapter II, avoiding the standard loop after the first two basement floors. Additionally, 
Sacrifice Rooms after 12 uses can be used at any point to teleport to the 
Dark Room, making it possible to see a proper ending and finish the run.
Fortune Telling Machine or the 
Rules Card.
Tainted Forgotten in the "SVPE RH0T" seed, time will only move when the skeleton moves.
Bug!
The Binding of Isaac: Rebirth
