It is well known that a canonical scalar field is able to describe either dark matter or dark energy but not both .
We demonstrate that a non-canonical scalar field can describe both dark matter and dark energy within a unified setting .
We consider the simplest extension of the canonical Lagrangian { \cal L } \propto X ^ { \alpha } - V ( \phi ) where \alpha \geq 1 and V is a sufficiently flat potential .
In this case the kinetic term in the Lagrangian behaves just like a perfect fluid , whereas the potential term mimicks the cosmological constant .
For very large values , \alpha \gg 1 , the equation of state of the kinetic term drops to zero and the universe expands as \Lambda CDM .
The velocity of sound in this model , and the associated gravitational clustering , is sensitive to the value of \alpha .
For very large values of \alpha the clustering properties of our model resemble those of cold dark matter ( CDM ) .
But for smaller values of \alpha , gravitational clustering on small scales is suppressed , and our model has properties resembling those of warm dark matter ( WDM ) .
Therefore our non-canonical model has an interesting new property : while the background universe expands like \Lambda CDM , its clustering properties can resemble those of either cold or warm dark matter .