Starting with the 3.2 release of MongoDB, you can get N random docs from a collection using the $sample aggregation pipeline operator:

// Get one random document from the mycoll collection.db.mycoll.aggregate([{ $sample: { size: 1 } }])

If you want to select the random document(s) from a filtered subset of the collection, prepend a $match stage to the pipeline:

// Get one random document matching {a: 10} from the mycoll collection.db.mycoll.aggregate([    { $match: { a: 10 } },    { $sample: { size: 1 } }])

As noted in the comments, when size is greater than 1, there may be duplicates in the returned document sample.

Do a count of all records, generate a random number between 0 and the count, and then do:

db.yourCollection.find().limit(-1).skip(yourRandomNumber).next()

Update for MongoDB 3.2

3.2 introduced $sample to the aggregation pipeline.

There's also a good blog post on putting it into practice.

For older versions (previous answer)

This was actually a feature request: http://jira.mongodb.org/browse/SERVER-533 but it was filed under "Won't fix."

The cookbook has a very good recipe to select a random document out of a collection: http://cookbook.mongodb.org/patterns/random-attribute/

To paraphrase the recipe, you assign random numbers to your documents:

db.docs.save( { key : 1, ..., random : Math.random() } )

Then select a random document:

rand = Math.random()result = db.docs.findOne( { key : 2, random : { $gte : rand } } )if ( result == null ) {  result = db.docs.findOne( { key : 2, random : { $lte : rand } } )}

Querying with both $gte and $lte is necessary to find the document with a random number nearest rand.

And of course you'll want to index on the random field:

db.docs.ensureIndex( { key : 1, random :1 } )

If you're already querying against an index, simply drop it, append random: 1 to it, and add it again.