diff --git a/src/compiler/crystal/codegen/codegen.cr b/src/compiler/crystal/codegen/codegen.cr
index ea51484396e9..9e887a10532b 100644
--- a/src/compiler/crystal/codegen/codegen.cr
+++ b/src/compiler/crystal/codegen/codegen.cr
@@ -424,10 +424,10 @@ module Crystal
         @last = int64(node.value.to_u64)
       when :i128
         # TODO: implement String#to_i128 and use it
-        @last = int128(node.value.to_i64)
+        @last = int128(node.value.to_i128)
       when :u128
         # TODO: implement String#to_u128 and use it
-        @last = int128(node.value.to_u64)
+        @last = int128(node.value.to_u128)
       when :f32
         @last = llvm_context.float.const_float(node.value)
       when :f64
diff --git a/src/compiler/crystal/codegen/llvm_builder_helper.cr b/src/compiler/crystal/codegen/llvm_builder_helper.cr
index 9136ead94247..1218214d2b46 100644
--- a/src/compiler/crystal/codegen/llvm_builder_helper.cr
+++ b/src/compiler/crystal/codegen/llvm_builder_helper.cr
@@ -21,7 +21,7 @@ module Crystal
     end
 
     def int128(n)
-      llvm_context.int128.const_int(n)
+      llvm_context.int128.const_int_of_string(n.to_s)
     end
 
     def int(n)
diff --git a/src/compiler/crystal/syntax/lexer.cr b/src/compiler/crystal/syntax/lexer.cr
index bc1ca2c9014c..0060bb460595 100644
--- a/src/compiler/crystal/syntax/lexer.cr
+++ b/src/compiler/crystal/syntax/lexer.cr
@@ -1557,7 +1557,7 @@ module Crystal
     def scan_hex_number(start, negative = false)
       next_char
 
-      num = 0_u64
+      num = 0_u128
       while true
         char = next_char
         if char == '_'
@@ -1576,7 +1576,7 @@ module Crystal
 
     def finish_scan_prefixed_number(num, negative, start)
       if negative
-        string_value = (num.to_i64 * -1).to_s
+        string_value = (num.to_i128 * -1).to_s
       else
         string_value = num.to_s
       end
diff --git a/src/int.cr b/src/int.cr
index 00454ab97774..b1c80c10feea 100644
--- a/src/int.cr
+++ b/src/int.cr
@@ -779,7 +779,7 @@ end
 
 struct UInt128
   # TODO: eventually update to literals once UInt128 bit support is finished
-  MIN = new 0
+  MIN = 0_u128
   MAX = ~MIN
 
   # Returns an `UInt128` by invoking `to_u128` on *value*.
diff --git a/src/llvm/lib_llvm.cr b/src/llvm/lib_llvm.cr
index 7c6fc5c333ef..4609066e234a 100644
--- a/src/llvm/lib_llvm.cr
+++ b/src/llvm/lib_llvm.cr
@@ -134,6 +134,7 @@ lib LibLLVM
   fun build_zext = LLVMBuildZExt(builder : BuilderRef, val : ValueRef, dest_ty : TypeRef, name : UInt8*) : ValueRef
   fun const_array = LLVMConstArray(element_type : TypeRef, constant_vals : ValueRef*, length : UInt32) : ValueRef
   fun const_int = LLVMConstInt(int_type : TypeRef, value : UInt64, sign_extend : Int32) : ValueRef
+  fun const_int_of_string = LLVMConstIntOfString(int_type : TypeRef, value : UInt8*, radix : UInt8) : ValueRef
   fun const_null = LLVMConstNull(ty : TypeRef) : ValueRef
   fun const_pointer_null = LLVMConstPointerNull(ty : TypeRef) : ValueRef
   fun const_real = LLVMConstReal(real_ty : TypeRef, n : Float64) : ValueRef
diff --git a/src/llvm/type.cr b/src/llvm/type.cr
index 01926eca8845..ad11b17c94be 100644
--- a/src/llvm/type.cr
+++ b/src/llvm/type.cr
@@ -129,6 +129,10 @@ struct LLVM::Type
     Value.new LibLLVM.const_int(self, value, 0)
   end
 
+  def const_int_of_string(string) : Value
+    Value.new LibLLVM.const_int_of_string(self, string, 10)
+  end
+
   def const_float(value : Float32) : Value
     Value.new LibLLVM.const_real(self, value)
   end
diff --git a/src/string.cr b/src/string.cr
index 5f2f4b3b1486..ff38eac59566 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -462,6 +462,38 @@ class String
     gen_to_ u64
   end
 
+  ### to_i128/to_u128
+
+  # Same as `#to_i` but returns an `Int128`.
+  def to_i128(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true) : Int128
+    to_i128(base, whitespace, underscore, prefix, strict) { raise ArgumentError.new("Invalid Int128: #{self}") }
+  end
+
+  # Same as `#to_i` but returns an `Int128` or `nil`.
+  def to_i128?(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true) : Int128?
+    to_i128(base, whitespace, underscore, prefix, strict) { nil }
+  end
+
+  # Same as `#to_i` but returns an `Int128` or the block's value.
+  def to_i128(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true, &block)
+    gen_to_ i128, ((1<<127)-1).to_u128, (1<<127).to_u128
+  end
+
+  # Same as `#to_i` but returns an `UInt128`.
+  def to_u128(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true) : UInt128
+    to_u128(base, whitespace, underscore, prefix, strict) { raise ArgumentError.new("Invalid UInt128: #{self}") }
+  end
+
+  # Same as `#to_i` but returns an `UInt128` or `nil`.
+  def to_u128?(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true) : UInt128?
+    to_u128(base, whitespace, underscore, prefix, strict) { nil }
+  end
+
+  # Same as `#to_i` but returns an `UInt128` or the block's value.
+  def to_u128(base : Int = 10, whitespace = true, underscore = false, prefix = false, strict = true, &block)
+    gen_to_ u128
+  end
+
   # :nodoc:
   CHAR_TO_DIGIT = begin
     table = StaticArray(Int8, 256).new(-1_i8)
@@ -485,13 +517,13 @@ class String
   end
 
   # :nodoc:
-  record ToU64Info,
-    value : UInt64,
+  record ToU128Info,
+    value : UInt128,
     negative : Bool,
     invalid : Bool
 
   private macro gen_to_(method, max_positive = nil, max_negative = nil)
-    info = to_u64_info(base, whitespace, underscore, prefix, strict)
+    info = to_u128_info(base, whitespace, underscore, prefix, strict)
     return yield if info.invalid
 
     if info.negative
@@ -509,7 +541,7 @@ class String
     end
   end
 
-  private def to_u64_info(base, whitespace, underscore, prefix, strict)
+  private def to_u128_info(base, whitespace, underscore, prefix, strict)
     raise ArgumentError.new("Invalid base #{base}") unless 2 <= base <= 36 || base == 62
 
     ptr = to_unsafe
@@ -523,7 +555,7 @@ class String
 
     negative = false
     found_digit = false
-    mul_overflow = ~0_u64 / base
+    mul_overflow = ~0_u128 / base
 
     # Check + and -
     case ptr.value.unsafe_chr
@@ -555,7 +587,7 @@ class String
       end
     end
 
-    value = 0_u64
+    value = 0_u128
     last_is_underscore = true
     invalid = false
 
@@ -608,7 +640,7 @@ class String
       invalid = true
     end
 
-    ToU64Info.new value, negative, invalid
+    ToU128Info.new value, negative, invalid
   end
 
   # Returns the result of interpreting characters in this string as a floating point number (`Float64`).