The geographical region denominated generically as Guianas encompassed the countries of French guyana, Surinam and Guyana. The whole region lies well within the tropics and therefore shares some general features: 
The single most important large-scale forcing is given by the ITCZ, which passes through the region twice a year and determines (along with local factors) the annual distribution of rainfall and winds.
Some characteristically tropical features present in all three countries include: a relatively high (25°C) mean annual temperature at the sea level , the diurnal range of temperature exceeds the annual range, and the annual temperature range is minimal (< 5° C). Most of the region has a wet marine rainfall climate.
 

As temperature show only small annual variations in the region, it is rainfall that determines the seasons in the Guianas.
And rainfall is largely determined by the influence of the ITCZ that passes through the region. The Atlantic ITCZ migrates from its northernmost position, at around 10° N during October to 5° N in January, and then to its southernmost position around April at about 5° S and back to 5° N in July.
The passage of the ITCZ twice a year through the region therefore leaves a semi-annual imprint in the rainfall distribution of the region, with most of the region having two wet seasons and 2 dry seasons with two exceptions. French Guyana has only one long wet seasons, due to its  Atlantic location , the expected minima during February and March is absent. The other exception is found in the southern part of Guyana, where there is only one wet seasons during northern summer.

In general the rainfall regime change westward from a single minima found in French Guyana to a double minima regime in Surinam and Guyana. The amounts of rainfall decrease from east to west, with the coastal fringe of French Guyana being the wettest and the western interior of Guyana receiving the less annual total amounts.

French Guyana, is the smallest and wettest of the three countries. It is also very low, with highest points lying below 300 m. There is only one rainfall regime with a wet and one dry season. A long wet season from mid November through mid August, with maxima in May. This maxima is associated with the passage of the ITCZ at the latitude of French Guyana in its annual migration. The remaining 12 weeks of the year are dry, when the ITCZ is its northernmost position. 
The country can be divided into three sub regions by vegetation. The coastal fringe is convered mostly by mangrove swaps and wetland and is the wettest region, with mean 3800 mm per year. The inland is covered mostly by rain forest and some savannah. The annual mean rainfall drops to 2500 mm, but the distribution is the same as at the coastal fringe.
Finally the interior of the country has mean altitude of 300 m and is covered by rain forest. The western part is drier than the eastern part.

Surinam has two wet and 2 dry seasons, associated with passage of the ITCZ in its annual migration. The wettest month are found in May-June when the ITCZ moves northward, the second wet season is found during December through January. The driest month are October and November. This rainfall distribution is found over the whole country, although the central highlands are the wettest, with an annual mean of 3000 mm. The coastal region receives about 2000 mm and so does the southern interior.

Guyana has a very similar rainfall distribution to Surinam with the exception to the Southern Plains of Guyana, where only one wet and dry seasons are found. The rainy season last from May to July, with the rest of the year being relatively dry. The tool amount of rainfall is about 1800 mmm per year and the vegetation is savannah.
 
 

Region	Rainy season	Dry season
French Guyana	Nov-Aug	Sept-Oct
Surinam and northern part and coast of Guyana	Apr-Jul  Sec max in Dec-Jan	Sep-Nov Sec min Feb-Mar
Southern Plains of Guyana	Apr-Aug	Sep-Mar