Seqtest: an R package for sequential triangular tests. The R package seqtest provides functions to perform sample size determination and a sequential triangular test for the expectation in one and two samples, probabilities in one and two samples, and the product-moment correlation coefficient. The main characteristic of the sequential triangular test is that there is no fixed sample size given in advance. That is, for the most recent sampling point, one has to decide whether (1) sampling has to be continued, (2) the null hypothesis is accepted, or (3) the alternative hypothesis is accepted, given specified precision requirements (i.e., type-I risk, type-II risk, and an effect size). In general, sequential triangular tests have the advantage that in many cases the average sample size is smaller than that of the corresponding fixed sample size tests. The use of seqtest is illustrated by testing a correlation coefficient’s null hypothesis: 0<ρ≤ρ0 using a sequential triangular test.