Substitute for marjoram

Marjoram is a type of Oregano, or rather it is from the Origanum family (origanum majorana, oregano is origanum vulgare). So oregano makes the closest substitute I find, though it is stronger, so use less, oregano is often called wild marjoram.

A great resource on spices is Gernot Katzer's website. The majoram page plus my own experience make me suggest sage (for the slightly bitter note) and thyme. I would not use oregano - to my Italian nose it smells very different.

Assuming you are just out of any types of marjorams (wild, oregano, otherwise) the spice coriander can be substituted in the less-herbal (where you would swing with sage) categories, i.e. poultry or eggs.

If you are adding marjoram to something with elements akin to smoked paprika, red pepper, here is where coriander comes in handy. I find it matches well with beef, venison, tomato particularly well. You need to tie the coriander's higher piques down though to emulate marjoram (which is more even in its notes); you can do this with thyme as suggested above, or maybe tarragon (though that can be pretty risky itself).

Not a close match, but sage can impart the same hint of smokiness, and I've used it from time to time.

I was looking for a substitute for marjoram for a herbal rice recipe. I used oregano and was very pleased with the outcome. It gave the rice an slightly earthy taste. Very good I must say.