diff --git a/lib/ecto/adapters/clickhouse/connection.ex b/lib/ecto/adapters/clickhouse/connection.ex
index 594a1dc..afaf2b6 100644
--- a/lib/ecto/adapters/clickhouse/connection.ex
+++ b/lib/ecto/adapters/clickhouse/connection.ex
@@ -860,9 +860,7 @@ defmodule Ecto.Adapters.ClickHouse.Connection do
     [?[, intersperse_map(list, ?,, &expr(&1, sources, params, query)), ?]]
   end
 
-  defp expr(%Decimal{} = decimal, _sources, _params, _query) do
-    Decimal.to_string(decimal, :normal)
-  end
+  defp expr(%Decimal{} = decimal, _sources, _params, _query), do: decimal_to_string(decimal)
 
   defp expr(%Tagged{value: value, type: :any}, sources, params, query) do
     expr(value, sources, params, query)
@@ -997,6 +995,10 @@ defmodule Ecto.Adapters.ClickHouse.Connection do
 
   @inline_tag :__ecto_ch_inline__
 
+  @max_decimal_precision 76
+  @max_decimal_output_digits @max_decimal_precision + 1
+  @max_decimal_coefficient Integer.pow(10, @max_decimal_precision)
+
   @doc false
   def mark_inline(param), do: {@inline_tag, param}
 
@@ -1171,7 +1173,7 @@ defmodule Ecto.Adapters.ClickHouse.Connection do
     [?', Date.to_string(date), suffix]
   end
 
-  defp inline_param(%Decimal{} = dec), do: Decimal.to_string(dec, :normal)
+  defp inline_param(%Decimal{} = dec), do: decimal_to_string(dec)
 
   defp inline_param(a) when is_list(a) do
     [?[, Enum.map_intersperse(a, ?,, &inline_param/1), ?]]
@@ -1226,10 +1228,9 @@ defmodule Ecto.Adapters.ClickHouse.Connection do
   # TODO Date32
   defp param_type(%Date{}), do: "Date"
 
-  defp param_type(%Decimal{exp: exp}) do
-    # TODO use sizes 128 and 256 as well if needed
-    scale = if exp < 0, do: abs(exp), else: 0
-    ["Decimal64(", Integer.to_string(scale), ?)]
+  defp param_type(%Decimal{} = decimal) do
+    {precision, scale} = decimal_precision_and_scale!(decimal)
+    ["Decimal(", Integer.to_string(precision), ?,, Integer.to_string(scale), ?)]
   end
 
   # context: https://github.com/plausible/analytics/pull/6049#issuecomment-3850062609
@@ -1261,4 +1262,63 @@ defmodule Ecto.Adapters.ClickHouse.Connection do
         "Map(Nothing,Nothing)"
     end
   end
+
+  defp decimal_to_string(decimal) do
+    decimal_precision_and_scale!(decimal)
+
+    if function_exported?(Decimal, :to_string, 3) do
+      apply(Decimal, :to_string, [
+        decimal,
+        :normal,
+        [max_digits: @max_decimal_output_digits]
+      ])
+    else
+      Decimal.to_string(decimal, :normal)
+    end
+  end
+
+  defp decimal_precision_and_scale!(%Decimal{coef: coef}) when coef in [:NaN, :inf] do
+    raise ArgumentError, "ClickHouse Decimal values must be finite"
+  end
+
+  defp decimal_precision_and_scale!(%Decimal{coef: coef, exp: exp})
+       when is_integer(coef) and coef >= 0 and is_integer(exp) do
+    scale = if exp < 0, do: -exp, else: 0
+
+    if scale > @max_decimal_precision do
+      raise ArgumentError,
+            "ClickHouse Decimal scale #{scale} exceeds maximum #{@max_decimal_precision}"
+    end
+
+    if coef >= @max_decimal_coefficient do
+      raise ArgumentError,
+            "ClickHouse Decimal precision exceeds maximum #{@max_decimal_precision}"
+    end
+
+    coefficient_digits = decimal_coefficient_digits(coef)
+
+    precision =
+      if exp >= 0 do
+        coefficient_digits + exp
+      else
+        max(coefficient_digits, scale)
+      end
+
+    if precision > @max_decimal_precision do
+      raise ArgumentError,
+            "ClickHouse Decimal precision #{precision} exceeds maximum #{@max_decimal_precision}"
+    end
+
+    {precision, scale}
+  end
+
+  defp decimal_precision_and_scale!(%Decimal{}) do
+    raise ArgumentError, "invalid Decimal struct"
+  end
+
+  defp decimal_coefficient_digits(coef) do
+    # The coefficient is already bounded to ClickHouse Decimal256 precision, so
+    # this allocates at most 76 bytes and keeps type inference straightforward.
+    coef |> Integer.to_string() |> byte_size()
+  end
 end
diff --git a/test/ecto/adapters/clickhouse/connection_test.exs b/test/ecto/adapters/clickhouse/connection_test.exs
index ac52d66..94b79bd 100644
--- a/test/ecto/adapters/clickhouse/connection_test.exs
+++ b/test/ecto/adapters/clickhouse/connection_test.exs
@@ -3467,6 +3467,113 @@ defmodule Ecto.Adapters.ClickHouse.ConnectionTest do
              "1,'a',true,'2024-04-12'::date,'2024-04-12 09:55:54.329788'::DateTime64(6,'Etc/UTC'),'2024-04-12 09:55:54'::DateTime('Etc/UTC')"
   end
 
+  test "decimal parameter boundaries" do
+    param = fn value ->
+      to_string(Connection.build_params(_ix = 0, _len = 1, [value]))
+    end
+
+    inline_param = fn value ->
+      to_string(Connection.build_params(_ix = 0, _len = 1, [Connection.mark_inline(value)]))
+    end
+
+    max_integer = String.duplicate("9", 76)
+    max_scale = "0." <> String.duplicate("0", 75) <> "1"
+
+    assert param.(Decimal.new("1.23")) == "{$0:Decimal(3,2)}"
+    assert inline_param.(Decimal.new("1.23")) == "1.23"
+
+    assert param.(Decimal.new("-1.23")) == "{$0:Decimal(3,2)}"
+    assert inline_param.(Decimal.new("-1.23")) == "-1.23"
+
+    assert param.(Decimal.new(max_integer)) == "{$0:Decimal(76,0)}"
+    assert inline_param.(Decimal.new(max_integer)) == max_integer
+
+    assert param.(Decimal.new(1, 1, -76)) == "{$0:Decimal(76,76)}"
+    assert inline_param.(Decimal.new(1, 1, -76)) == max_scale
+  end
+
+  test "decimal literal boundaries" do
+    literal = fn decimal ->
+      {query, params} = plan("schema" |> select([], true), :all)
+      query = %{query | select: %{query.select | fields: [decimal]}}
+
+      query
+      |> Connection.all(params)
+      |> IO.iodata_to_binary()
+    end
+
+    max_integer = String.duplicate("9", 76)
+    max_scale = "0." <> String.duplicate("0", 75) <> "1"
+
+    assert literal.(Decimal.new(max_integer)) == ~s[SELECT #{max_integer} FROM "schema" AS s0]
+    assert literal.(Decimal.new(1, 1, -76)) == ~s[SELECT #{max_scale} FROM "schema" AS s0]
+  end
+
+  test "decimal parameters reject over-limit values before rendering" do
+    param = fn value ->
+      to_string(Connection.build_params(_ix = 0, _len = 1, [value]))
+    end
+
+    inline_param = fn value ->
+      to_string(Connection.build_params(_ix = 0, _len = 1, [Connection.mark_inline(value)]))
+    end
+
+    assert_raise ArgumentError, ~r/precision 77 exceeds maximum 76/, fn ->
+      param.(Decimal.new(1, 1, 76))
+    end
+
+    assert_raise ArgumentError, ~r/scale 77 exceeds maximum 76/, fn ->
+      param.(Decimal.new(1, 1, -77))
+    end
+
+    assert_raise ArgumentError, ~r/precision exceeds maximum 76/, fn ->
+      param.(Decimal.new(String.duplicate("9", 77)))
+    end
+
+    assert_raise ArgumentError, ~r/precision 77 exceeds maximum 76/, fn ->
+      inline_param.(Decimal.new(1, 1, 76))
+    end
+
+    assert_raise ArgumentError, ~r/scale 77 exceeds maximum 76/, fn ->
+      inline_param.(Decimal.new(1, 1, -77))
+    end
+
+    assert_raise ArgumentError, ~r/ClickHouse Decimal values must be finite/, fn ->
+      inline_param.(Decimal.new("NaN"))
+    end
+
+    assert_raise ArgumentError, ~r/ClickHouse Decimal values must be finite/, fn ->
+      param.(Decimal.new("NaN"))
+    end
+
+    assert_raise ArgumentError, ~r/ClickHouse Decimal values must be finite/, fn ->
+      param.(Decimal.new("Infinity"))
+    end
+
+    assert_raise ArgumentError, ~r/ClickHouse Decimal values must be finite/, fn ->
+      inline_param.(Decimal.new("Infinity"))
+    end
+  end
+
+  test "decimal literals reject over-limit values before rendering" do
+    literal = fn decimal ->
+      {query, params} = plan("schema" |> select([], true), :all)
+      query = %{query | select: %{query.select | fields: [decimal]}}
+
+      query
+      |> Connection.all(params)
+      |> IO.iodata_to_binary()
+    end
+
+    assert_raise ArgumentError, ~r/precision 77 exceeds maximum 76/, fn ->
+      literal.(Decimal.new(1, 1, 76))
+    end
+
+    assert_raise ArgumentError, ~r/scale 77 exceeds maximum 76/, fn ->
+      literal.(Decimal.new(1, 1, -77))
+    end
+  end
+
   # https://clickhouse.com/docs/en/sql-reference/data-types/int-uint
   test "integer boundaries" do
     import Bitwise