Serbia Under-21 international Dmitrovic, 22, has joined the Addicks on an 18-month deal.
Scotland Under-21 international Watt scored two goals in 17 appearances for Standard Liege after moving from Celtic last summer, having previously spent the 2013-14 campaign on loan in Belgium with Lierse SK.
"He's a striker who will add something to our team," Charlton head coach Bob Peeters told the club website.
"He's a guy who can score goals, he has a lot of pace and although he's only 21 he has a lot of experience already and has played in the Champions League.
"He didn't have his best period at Standard, but I think now that he's back in a country where they speak English, and he feels at ease, he can do the job."
"We have seen a lot of Marko and been scouting him for eight or nine months," goalkeeping coach Ben Roberts said.
"He is over here to compete for his place and also prepare for the European Under-21 Championship in the summer.
"He has trained for us over the past few days and made a really good impression already, so I'm looking forward to spending some time with him."