JsonSerializer - serialize decimal places with 'N2' formatting

You'll have to write your own custom JsonConverter and use it to intercept the decimal type so you can change how it gets serialized. Here's an example:

public class DecimalFormatConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(decimal));
    }

    public override void WriteJson(JsonWriter writer, object value, 
                                   JsonSerializer serializer)
    {
        writer.WriteValue(string.Format("{0:N2}", value));
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override object ReadJson(JsonReader reader, Type objectType,
                                 object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();    
    }
}   

To use it, simply pass in a new instance of this custom converter to the SerializeObject method:

var json = JsonConvert.SerializeObject(yourObject, new DecimalFormatConverter());

The accepted answer is correct, but expanding upon the comments on accepted answer:

If you want the decimals in your JSON to be numbers instead of strings, you need to use WriteRawValue and use :0.00 instead of :N2 for the string formatting (as N2 includes thousand separator commas and other culture specific number formatting that will break your JSON)

public class DecimalFormatConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(decimal);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteRawValue($"{value:0.00}");
    }

    public override bool CanRead => false;

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

This is the difference in output compared to the accepted answer.

# writer.WriteRawValue($"{value:0.00}");
{
    "MyDecimal": 3.50,
    "MyBiggerDecimal": 12345.50
}

# writer.WriteValue($"{value:N2}");
{
    "MyDecimal": "3.50",
    "MyBiggerDecimal": "12,345.50"
}

Note - the accepted answer is correct for the OP's specific question i.e. serialize 3.5 to "3.50", but I got here wanting to serialize 3.5 to 3.50 (without the string quotes).