Transforming single QgsGeometry object from one CRS to another using PyQGIS?

use QgsGeometry.transform( QgsCoordinateTransform tr ). for example after created your instance of QgsCoordinateTransform with source and dest crs, for each geometry instance do:

sourceCrs = QgsCoordinateReferenceSystem(4326)
destCrs = QgsCoordinateReferenceSystem(2154)
tr = QgsCoordinateTransform(sourceCrs, destCrs, QgsProject.instance())
myGeometryInstance.transform(tr)

https://qgis.org/api/classQgsCoordinateTransform.html#aa5ad428819ac020f8f5716e835ab754f

beware that the transformation (applying QgsCoordinateTransform) will change the QgsGeometry instance.

Old Question, but duckduckgo brings you here for "pyqgis transform single geometry"

Just like Riccardo Pointed out, the accepted answer does not work for scripts in QGIS 3, the situation is explained in the api reference:

Python scripts should utilize the QgsProject.instance() project instance when creating QgsCoordinateTransform. This will ensure that any datum transforms defined in the project will be correctly respected during coordinate transforms. E.g.

transform = QgsCoordinateTransform(QgsCoordinateReferenceSystem("EPSG:3111"),
                                   QgsCoordinateReferenceSystem("EPSG:4326"), QgsProject.instance())

Using QgsCoordinateTransform in QGIS 3 will transform the geometry instance inplace, and return 0:

geom = QgsGeometry(QgsPoint(5,20))
sourceCrs = QgsCoordinateReferenceSystem(4326)
destCrs = QgsCoordinateReferenceSystem(2154)
tr = QgsCoordinateTransform(sourceCrs, destCrs, QgsProject.instance())
geom.transform(tr)
#[out]: 0

geom
#[out]: <QgsGeometry: Point (930236.95432910311501473 3567516.9810206750407815)>

back_tr = QgsCoordinateTransform(destCrs, sourceCrs, QgsProject.instance())
geom.transform(back_tr)
#[out]: 0

geom
#[out]: <QgsGeometry: Point (5 19.99999999999911182)>

If you want to copy an already existing QgsGeometry Instance, the QgsGeometry class constructor will make the trick. From the docs:

Class: QgsGeometry class qgis.core.QgsGeometry Bases: sip.wrapper

Constructor

QgsGeometry(QgsGeometry) Copy constructor will prompt a deep copy of the object

example:

geom2 = QgsGeometry(geom)
geom2
#[out]: <QgsGeometry: Point (5 19.99999999999911182)>
geom
#[out]: <QgsGeometry: Point (5 19.99999999999911182)>
geom2.transform(tr)
#[out]: 0
geom2
#[out]: <QgsGeometry: Point (930236.95432910579256713 3567516.98102056747302413)>
geom
#[out]: <QgsGeometry: Point (5 19.99999999999911182)>