diff --git a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGamePacket.java b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGamePacket.java
index 645dc7b6..31b03911 100644
--- a/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGamePacket.java
+++ b/proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGamePacket.java
@@ -245,7 +245,17 @@ public class JoinGamePacket implements MinecraftPacket {
     if (version.noLessThan(ProtocolVersion.MINECRAFT_1_9_1)) {
       this.dimension = buf.readInt();
     } else {
-      this.dimension = buf.readByte();
+      // Vanilla 1.7.10 uses a signed byte for dimension (-1 Nether, 0 Overworld, 1 End).
+      // Modded servers hack this to an unsigned byte to allow dim IDs up to 255.
+      // We must store the canonical int here because RespawnPacket.fromJoinGame copies this
+      // value into a packet that encodes dimension as a 4-byte int (RespawnPacket.encode,
+      // pre-1.16 logic). Sign-extending a modded byte like 0xB4 to int -76 sends an
+      // illegal dimension ID to the client and crashes it.
+      //
+      // The wire byte is ambiguous: 0xFF could be vanilla Nether (-1) or modded dim 255.
+      // We resolve in Nether's favor since vanilla 1.7.10 only ever uses -1/0/1.
+      short raw = buf.readUnsignedByte();
+      this.dimension = raw == 0xFF ? -1 : raw;
     }
     if (version.noGreaterThan(ProtocolVersion.MINECRAFT_1_13_2)) {
       this.difficulty = buf.readUnsignedByte();